Difficulty topping up at some LU ticket machines

Yesterday I tried to top up my Oyster card at a ticket machine at King’s Cross St Pancras Underground station. I tried to use both my debit card and my credit card on two different machines, but both times both cards were rejected and I got a message on the screen saying my card issuer had declined this transaction. One of those machines had a banknote slot but had a red light on the slot to show that it wasn’t accepting notes.

I have a colleague who had the same problem recently both at King’s Cross St Pancras and at Ealing Broadway. Both he and I have first generation Oyster cards.

I then tried to top it up at a National Rail ticket machine on King’s Cross main line station, and it worked fine.

I wonder if this is just a temporary glitch in the system, or maybe TfL is deliberately stopping first generation Oyster card holders from topping up by credit or debit card (or has programmed its ticket machines to randomly reject such payments) to try to make us all upgrade to a second generation Oyster card or switch to using contactless?