You didn't mention the make of vehicle, and if its manual or power windows.
Basically, you are getting binding, which can be caused by debris or even looseness of the mounting bolts. Either way, the door panels will have to come off. The little plastic rollers in the track may be worn out also. If, when rolling the windows, it feels like a shuddering movement or tugging, the roller may have developed a flat spot. Pulling the mechanism isn't that hard, and it makes cleaning way easier. Depending on the vehicle, it may be a few bolts (and two on the glass) holding it in. May also be held in with pop rivets, as on my 90'. Just make sure you hang onto that glass, because it will drop quick! Once cleaned, lubricate with chassis grease (per Haynes).
If it happens to be power windows, there is a little more to it.