Chain issues with drive train

Drive Train Issues

After using R2 for a full weekend event, there was tons of drive slippage
"The sound it makes similar to changing gears on a 10speed bike" and its horrendous...  Many times preventing R2 from moving forward correctly.

Once we got him home, removed the outer feet and disassembled the shells from the motor mounts.

Potential issues:

1. There is maybe too much slop in the chain (although we cannot tighten it any more or move it any more on the links.
2. The gears are loosing grip and letting the chain jump off the gear

I removed the retaining clip on the chain and tried to tighten, there is not enough slack to go another link.

Looking for answers