Extracts are already converted. You don't add enzymes to them. Malted grains have to be converted in the mash tun.
Most under attenuated beers are that way due to inadequate yeast pitching rates. Pitch enough yeast and keep it happy/healthy and you'll reach the potential you should be reaching.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10