You may laugh at this, but the problem could well be in your jab and how you throw it. Let me explain.

Lots of times, especially when a guy is really concerned about his straight punch, he'll cheat his jab. By that I mean he'll paw with it or push it, and not fully step in with it, all in anticipation of really putting everything on the punch that comes behind it. This will mess up your distance and, as a result, you'll tend to smother your own punch. Typically this will happen with an orthodox fighter on the heavy bag, but I can see how it would happen to a southpaw while working on the pads, depending on how your trainer moves.