ATL has gotten harder to redeem from ... the Chase sign-up bonuses are probably one factor (that has been on and off over the past few years), but I noticed a big change in seat availability after they started letting AA folks redeem on BA transatlantic.

That said, I have two upcoming award bookings out of ATL in F in the coming months, both using 2-for-1 certificates, so I can't complain about that. I can only complain about frustrations over MFU/UUA availability from WT+ to CW.

For the OP, I'm curious at what stage the transaction failed. The last part when it went to authorize your credit card? Or earlier?

A few years ago, I had consistent problems booking on BA's web site. The problem turned out to be that I was not registered in the Verified by Visa program. Once I did that, I had no problem. (And oddly enough, for my last few bookings, I never get asked for my Verified password, it just pops up and goes away automatically without asking.)