I'm still new at this myself, but based purely off of what I've read:
1. The canned kits really just aren't that good to begin with.
2. The canned kits often come with crappy, old yeast, so if you used the yeast that came with the kit that could be a big part of the problem.
I'm fermenting my first batch now, it was a kit that was assembled by the local homebrew store, still good to learn the basics with but with considerably better ingredients.