possibly the motor...had the same problem with my driver's side. It would go up but slow and shaky. Mine seems to be getting better. Does it make any noises at all? Does it sound like its trying to roll up or just nothing at all?
Well I never tried this but put some pressure on it ( up with your hand) and run it up.
On the other hand if it isn't down all of the way try running it down (put pressure down with your hand at the same time)
Mine is doing the same thing but it's actually the switch on the drivers door. Have you tried using the switch on the pass door? Mine works fine with the switch on the pass door but if I use the one on the drivers door I have to try pushing/ pulling on it at different angles to get it to work.
It's not the regulator, the window just needs oiling. You need to put some upwards pressure on it, and it'll roll up slowly. Also while putting upwards pressure on it, pull it forward towards the engine to relieve the stress.
The weather stripping that guides the window up becomes very dry and hinders the window from rolling up. You also might notice it has more of a harder time rolling up after it rains.
My drivers side has been doing this for a while now, and I just keep oilling it. Just don't ever put it all the way down, so you have some of the window sticking out to grap onto and pull upwards.