The first few times Transferwise insisted it's a broken card reader, then a few calls and emails later it became the merchant's terminal that's at fault.
On some of the calls, Transferwise simply hung up on me when I didn't take "you should try it again" for an answer.
Finally after escalation it became the terminal's providing bank, which didn't recognize my Transferwise card number (issued in New Zealand).
Meanwhile other people used their Transferwise cards at the same locations mine failed. So I requested a European version of my Transferwise card but TW insisted that would break their contract with Mastercard or something.
3 or 4 weeks later, after I had already left Bulgaria, they emailed to update that my card would now be working.
Needless to say, I always travel with more than 1 provider's card with me, and this runaround from Transferwise is why.