It's gonna be pretty hard to tell you what is broken based on a couple sentences. What I would suggest is that you slowly disassemble the gearbox (while taking lots of pictures) to find the problem.
Then, you would need to find out how to fix the problem. At this point, we may be able to help you or it may not be something that can be fixed. Then you would need to source a replacement part and install it. After all that, it will likely break again.
I wouldn't suggest using copals in a competition bot, but if you want to learn then try and fix them.