Typical game is 6-handed, $20 buyin, NL, .25/50

As I've been playing over the last few years or so, I'm discovering that I'm only a slightly above break-even player. Not making much money at all.

For the most part, it seems like most sessions I'm very near even (give or take 15%)

The only big wins I ever see are usually on double-ups from monster hands where the other person just can't let go. However, these are rare, of course.

Where would my biggest leak be?

In short-handed play, where is the majority of your profit coming from?

Top pairs?
Being tricky with pocket pairs to catch sets?

Confused......