Couple of questions to see where you are currently at and why.
Is feedburner the only method of pushing your rss feed?
Is it linked to real content or are you stuffing it with crappy ad pages and affiliate junk?
You need real, simple and targeted content updates, which you can get automatically from any given source and have no problems getting your subscriber list up into the 300-500 average readership.
You have to use headlines, fresh articles and press release content, as well as information these people want to read about. That's the whole reason they are onboard with your feed in the first place...you offered them something they liked at one point and they are just skimming through your current junk to find another diamond in the rough.