I’ve shopped online with Morrison’s using my Amex with no problems, but when I changed my payment details to Monzo they want to see additional ID. I’ve never had this before. Do they think I’m a fraudster? 
Give them a call and ask.
I’m sure they have a legitimate reason and it will save us all from speculating on a bunch of stuff that it “could be” 
Report back here when you do so we can all learn from it 
Probably just a random security check due to you using different payment details than usual.
Probably just because you changed it - doesn’t matter that you changed it to Monzo or whoever
Hey, in case it eases your mind at all I’ve never had a problem between Monzo & Morrison’s online, I order from them regularly. Coincidentally, I also switched from an Amex to Monzo when I last changed it too.
Yo, I know this is 9 months later, but in case anyone else comes here looking for this question (I know I did).
I’ve been having problems with my Monzo card on Morrisons online for months. It would come up as card activation check on my phone, I’d approve the check… nothing would happen and the checkout would tell me there was an issue with my card.
Finally fixed it today. Outside of the checkout, on My Account > Account Settings > Payment Details, I added my Monzo card as a payment option (deleting the existing one it said there was a problem with). I was then able to do a full card check through the Monzo app.
Then I could go back to checkout and select that payment method, after which the card verification check went through without issue.
I’d love to know if that works for anyone else!