If the motor is not moving around in the case, the only thing that could cause your problem is having something out of round or a chain link that is not moving freely (this assumes you have the correct pitch chain for the sprockets and no damaged drive links dragging in the bar). I'd start by finding the tightest point, then marking a spot on both the sprocket and the tip sprocket. Then carefully move the chain forward until you hit the tight point again and re-check those two sprocket marks. Since they are different sizes, they will not rotate equally, and if one shows your mark in the same spot it was at the first tight point, you have found your problem. Of course, that tight point should be repeatable with every turn of the suspect sprocket to the same position.
If it indicates the rim sprocket, then I would pull it off and move it 180 degrees to repeat the test. If the marked spot continues to be at the same place for each tight point in the chain, then it confirms the sprocket, but if it is now reversed, then the problem is with the output shaft. or clutch drum. Of course, since the output shaft is not turning at all when you manually move the chain, that makes it HIGHLY unlikely that it could be the issue. But the clutch drum COULD be the problem, and that would be shown by moving the rim sprocket on the drum.