Probably a physical obstruction, likely a bit of plastic from the molding process. I had a similar problem and ended up cutting out the bottom of the slot below the tip of the flipper to stop it from dragging and getting stuck to one side. If you can tilt the track to one side and the flipper moves, but the same amount of tilt the other way won't move it back, or even more tilt won't move it back, then it's definitely a physical problem.
If it moves freely just by tilting the track, then it might be a bad solenoid on the one side, or some other electronic issue.
If it moves freely just by tilting the track, then it might be a bad solenoid on the one side, or some other electronic issue.