This is usually due to over loading the roller, which, in turn, causes improper leveling of the paint on your surface. Once your roller has been saturated in the well of your paint tray, roll it up and down repeatedly on the grid portion of the tray to ensure any part of the roller that was over or under loaded will be evened out. The first roll onto the wall should be in the shape of an "X" - then roll out the "X" until you need to reload. If you simply start out on the left and work to your right until you run out, the first pass will always release the most paint, giving you uneven results.