Rules for 2nd Queen

My opponent gains a second Q from pawn promotion, which goes on to take my Q. Then I take his 2nd Q with a Knight but I am not credited with either the Q capture or a pawn. As if the capture move never happened - why is this?

After the move bxa1=Q you had "gained" a pawn on the sideline.

The system won't recognise the Queen "capture" as your opponent still has a Queen on the table.
You instead "gained" a pawn, which was shown after the promotion was made.

Thanks for explaining. I think I made a very stupid move anyway. Will think more carefully in future.