Diagnosing Belt Skip Under Load

[Monochron]

Our belt driven intake arms are currently experiencing a lot of skipping (ratcheting) when accelerating and decelerating. Our arm pivots on a shaft that stretches the front of our robot and is driven by two Bag motors + gearboxes. A 18 tooth pulley from the gearboxes drive a 36 tooth pulley on the shaft/arm. The full arm can be seen in the video below.

What seems to be happening is that when the arm drives to drive the gearbox, even when unpowered, it often skips the belt on the smaller pulley. So when we are powering the arm, it skips when trying to stop because the momentum of the arm is trying to drive the small pulley.

The tension of the belts seems reasonable (one belt is slightly tighter than our KOP drivetrain belts, and the other is slightly less tight [resulting from fab inconsistencies]), though adding tensioners is possible. As best as I can tell our alignment from pulley to pulley is fine.

Can anyone comment on possible causes or suggest ways to fix this? We expected the belts to stop and stall the motors before skipping. Right now this is making PID control of the arm pretty much impossible, making it very very difficult to accurately control it and intake balls. About a week ago we made a video of the problem to ask a couple of friends, you can watch that video here.

Any advice is greatly appreciated.