This has happened to me on two separate bookings in the last six months, but in both cases it was after a flight was changed.

The first I discovered only when trying to check in last week, as there was nothing additional to pay - BA had cancelled the original flight and moved us but apparently never reticketed. I’d actually tried to get my e ticket online but the message said it wasn’t available due to the kind of booking, so I assumed the cancellation meant I couldn’t access it. Had to call BA and get them to issue the ticket, I never actually got an email but could then check in.

On the second, I made a change to the flight in mid October and gave all the payment info for the additional fee. Got an email from BA two weeks later at start of November, had to give CVV again, then two weeks later again in mid November, this time they needed the billing address. I did receive the ticket almost immediately then.

Needless to say I’m now checking all my bookings!