Try bringing your line back further behind you before you start your roll cast. The further back you start; within reason, the easier it will be to get your roll cast really working for you. Also, on a roll cast, I will often times haul the line on the forward cast; the extra line speed that you generate with the haul will show up as extra distance when you complete your forward cast.
As others have said, make sure all the slack is out of your line before you start your roll cast and don't point your rod at the water at the end of your cast; the line will go where the rod tip is pointed. If you point it down at the water, you'll get a short roll cast, that will end up, essentially, where it already is. Keep it at least horizon level when you abruptly end your forward cast.
Work with shorter distances at first (I personally think that 30" is a good distance to start with), then as it starts to really work for you, stretch it out. You'll be surprised how far a good roll cast will actually go; it's, essentially, a half-cycle water haul.
Have fun with it!