We need serious problems with two monzo cards whilst in India. We though that the Monzo card was designed for travelers? Everytime our cards were refused the message stated that the card supplier could not be contacted so we used our normal cards (nationwide, coop), they always worked
This car let us down really
What message did you get in the app? Were the cards you used instead Mastercard or Visa?
Without seeing the decline codes on your account it’s difficult to know exactly what went wrong, but if its an issue with the card then send us message in the in-app Chat and we can take a closer look.
Did you switch on the magstripe on your card? By default its switched off, but this post shows what you need to do to turn on, especially for travel in India!
India is on a very complicated banking infrastructure so it really depends which part of India are you visiting (and trying to use Monzo). As mentioned previously, switching to magstripe helped me in Mumbai. In New Delhi, I didn’t have to do anything whereas in North Eastern States, it didn’t work at all. FYI, it worked for me successfully at ICICI Bank, SBI Bank & Kotak Bank ATM Machines which are widely available throughout India.
India cash machines are confusing. After a lot of trial and error I seemed to have less problems with ICI cash machines but even then you seemed to have two types of cash machines “national” enabled and “international” enabled. So even different cash machines in the same branch gave different results 
What I figured out was that the cash machines seem to have 2 modes of operation. Pit card in and remove the card and it will attempt to use the magstrip. If you put it in and leave it in, it will attempt to use the chip, but you have to wait 30 seconds or more for the chip to pick up.
As I was there on business I gave up in the end and switched to using my credit card in India and had zero problems.