I'm assuming this is a hang on back. Is the impeller spinning?
Sounds like it's either not creating enough of a siphon (could be because you don't have it together tight enough, make sure it's seated well, also check to make sure there's no cracks or anything in the top of the tube) or the impeller isn't spinning fast enough to suck any water in. That could be because the impeller is damaged or something is creating friction so it can't get going right. Otherwise maybe the motor is bad, it might sound like it's running but it's not enough to run properly.
Try lifting and reseating the tube several times while it's plugged in and the back is filled with water. I've succeeded in getting stubborn filters running many times with that method. Sometimes the tube will either not seat properly the first time or traps air preventing it from priming.
If that doesn't work take it apart again and make sure everything is in right and nothing got in the filter while you were reassembling it (I've had a strand of hair get in an impeller when I put a filter back together before and it was enough to stop it from working).
Other than that I don't really have any suggestions except maybe trying a new impeller. Of course if that fails you're out the money you spent on the impeller, but if it works at least you're not looking at buying a new filter.