It takes a lot of practice in converting these advantages. I recommend puting the games into a computer for analysis and seeing where you went wrong and how the computer says you should convert the edge.
Sometimes you can have an advantage--a pawn or an exchange--and it's just tough as hell to squeak out that extra half-point.
I am about 1800-1900 USCF, but I'm stuck there because though I never lose to lower rated players, I can't beat them either. It seems that the only way I can beat them is when they blunder or make huge innaccurices. And when I get "positional advandtages," I can never convert them. Any ideas?