I've had the same problem with a couple of keys on my Bold and I cured it as follows:-
I followed the Youtube video tutorial here
that shows how to take the phone to pieces.
You will see that there is a thin white plastic film stuck to the circuit board that the keys press on to.On the reverse side of this film are round silver contacts that press onto copper contacts on the circuit board when you press a key so using something like a craft knife or scalpel gently ease up the edge of the film untill you can get hold of it.As it's just your "A" key,you won't need to completely remove the white plastic film (I did),you will only need to lift it enough so that you can clean the contacts on the film and the board that are used when you press that key.Any mild solvent should do the job,having nothing else I used ordinary white spirit that you clean houeshold paint brushes with on a cotton bud,then dried everything with a dry one.
Put it back together and hopefully you will have a fully functional keypad again.