The bolt is designed to lose a small amount of air through the back cap with every cycle. If you think it is losing too much then you need to replace the small o-ring at the very end of the bolt 'tail' it is a 007 BN o-ring. Check your manual if you are not sure.
Is it the stock backcap? Had a few problems with mine so i changed it had non since also have you taken the bolt out lately ? If so did you make sure everything was aligned properly when you slid it back into place? There's a small sticking out bit that needs to be aligned properly, I'm sure a more technical person could describe it better but still :E
Hope this helps