Haha - I was at the San Mateo Caltrain station earlier this year with the exact same issue. Interestingly, I seem to recall that my Santander wasnāt accepted either - ended up paying with a credit card.
Headed back to the Bay Area in a few days, should have far less acceptance issues now 
Awh man. Hope you have a blast! I know one place where the card works just fine, and thatās the Mikkeller Bar on Mason St, San Francisco 
Out of curiosity, how can I easily find out which version of the Current Account preview debit card I have so I bear this in mind for next time?
If it doesnāt have a name on it at all, thatās the first version of the current account card. Ideally you should replace this one if you have it, as it doesnāt support offline-only transactions or contactless magstripe which is used a lot in the US.
If it says āMonzo Bankā instead of the name, thatās the second version, and if it says your name on it thatās the third and final version. Spec wise thereās no difference between the two - I use the āMonzo Bankā one myself as I canāt be bothered to change it again at the moment 
Ooh thatās right you had cards with no name on themā¦ Was there a name (fake name) in the track 1 data on the magstripe?
If not, that could have been the problem. I know, I knowā¦ But Iāve definitely seen that cause issues in the US before.
Is there anything monzo can do about not been accepted on the New York subway / long island railroad road system. Apart from that and contactless everything else worked fine. However the subway issue did cause a fair few frustrations (especially when you read people saying 'type 99999: or ātype 10001ā and nothing works )
For LIRR I just buy them in their app now. Saves queueing.
Going to New York in Feb so be good to see a solution!
Did you try doing what Mastercard actually recommends (digits from your postcode followed by trailing zeros)? Iām not convinced it will work any better, but Iām curious if you tried it.
Just landed in New Orleans. Taxi from airport. Monzo 1 Curve 0
I went to America over the Thanksgiving period and took my monzo with the aim being to use it as my main card. Iām so glad I did as it was extremely easy.
First of all, not once was I asked about DCC. Everywhere I went was automatically processed in $, not sure if this is because I wasnāt in a high tourist area or if itās something Mastercard or Monzo have worked on? 
Secondly, I only had my monzo card actually declined once, now this is where I hope monzo team can work on an update, as the app didnāt tell me why it was declined, so I had to use my legacy card;
As well as my card being declined there, the map is horribly wrong, not sure if just a glitch or what.
The other place I had issue with my card was trying to order Pizza Hut Delivery whilst there. The app told me incorrect expiry date, so I suspect the issue was due to date formats perhaps?
Everywhere else took my card fine either with contactless - or ātapā as it was called, Chip and PIN, or swipe.
My only further feedback would be the option to add information about places. I noticed that nearly everywhere I went had no image, except for Charleyās Steakery and Krispy Kreme, even down to common places such as T-Mobile and Gamestop having no icon.
I really liked being able to track what I spent in both $ and Ā£, it was highly useful for tracking my budget.
So overall Monzo worked amazingly whilst I was over in the States. Better than my legacy card, quick story; above I mentioned using it at a Coke machine, despite telling my legacy building society that I would be in Michigan, they saw that transaction and locked my card until I went into a UK Branch - good job I only needed to use it that once!
Iām hoping Iāll be back to the States again, and monzo will definitely be my go to card! 
Was that a Coca-Cola Vending machine with one of the old swipe and magstripe contactless readers they use? Those things decline a ton, even with US-issued cards. Only some cards work with them, using contactless.
Try swiping if you really want to use one, thatās more likely to work.
I just wonder why they would decline, as in this case it seems like the transaction was sent to Monzo just fine. If it was a faulty reader Iād assume it wouldnāt even reach out to the acquirer bank to authorise the transaction.
I donāt know but I always figured there was something about the authorisation message from them that banks donāt likeā¦ itās not random. Some banks always approve, some always decline.
@RichardR - perhaps you could comment why so many cards decline at US Coke vending machines? Itād be interesting info if youāre able to. I figure there must be something really odd about them to decline so often, and to actually cause one of @aimeec110ās banks to completely lock out their accountā¦ (though thatās really never acceptable to strand someone abroad).
I would assume a problem with authorization messages. It even happens with ATMs. I have a local ATM here in Essex (Cashzone) that is always declined by a Dutch bank due to an incorrectly formatted message.
If you do get such a decline you can ask support and they will give you the decline reason as described by the Monzo card processor 
Cool, Iāll see if itās still happening next time Iām in the US.
Not even swipe worked, but worked with legacy. Itās bizarre. Legacy claimed when I was locked out it was due to the ālow security of the transactionā whatever that means!
It could just mean contactless magstripe mode, but I think thereās more to it than that given how many even American banks decline transactions from these machines when they allow other contactless magstripe transactions. Have you asked Monzo the decline reason?
Also, your legacy bank punishing you by locking your account out for the rest of your trip because you dared use your card at the worldās largest beverage companyās machine is just over the top. To me, thatās unacceptable. Imagine if you were one of the people who only has one cardā¦
Can I ask - Iām here in America now and Iāve tried to take cash out a few times but Iāve noticed that each ATM Iāve tried charges a fee (ranging from $3-8)- I know this isnāt a monzo cost (I think itās free up to $200?) but is from the bank / ATM - did you find anywhere where it was free to take cash out, please? Thank you in advance! Iāve got my monzo which I am using but Iāve noticed that people definitely prefer cash and cards are definitely not used as widely hereā¦ thank you!
from the "monzo in the USA " wiki thread
Most ATMs charge to withdraw cash (approximately $3 but can be up to $6). Reports suggest TD Bank, Wells Fargo and Citibank might be free. Another person says āthe best I found were the Allpoint ATMs that are located in McDonalds or Western Union that only charged $1.19 a timeā.