An incomplete fermentation can also come from bad yeast. If they're not up to the task, they can leave some sugars uneaten (that's what it sounds like happened to you), or just make a messy job of it.
Did the LHBS guy take a pack of yeast out of the refrigerator and hand it to you, or was it taped to the kit? Was it a recognised brand? Was there a date on it?
Your yeast is all that's making that sugar-water into beer. Treat it nice. Go for a trusted brand: Safale or Nottingham at least, but if you want the very best Wyeast or White Labs. If it's dry yeast, proof and rehydrate it first. If it's liquid yeast, make a starter. Keep it at a steady temperature, preferably (for a 'clean' profile) toward the bottom of its fermentation range. And if you feel you must take your beer off of it, at least give it a few days to clean up after itself (after the bubbles stop). Trust me.
If you're making a lager next, I'd highly recommend a liquid lager strain. If you can't do lager temperatures, try a California lager strain.
Good luck.
Kai