When I teach sit I hold the treat up over there heads where they can see it. Once they are paying attention I give the command and slide the treat up over their head - which usually puts them into a sit. If just using the treat as a lure doesn't help, you can also push their butts down while using the treat. Also, the method I use teaches them a hand signal as well. Travis will now sit (and do other commands) with just hand signals. (and no treats as lures).

I hope that made sense.