Yep hello, I had this problem when using PLA sprockets, after a while the motors got hot and the plastic softened a little and they started spinning.

I cut a groove on the shaft, just shallow, and used some 'serious glue' which is a flexible glue, it gripped ok, and that worked fine. I thought the groove would create a good 'key' for the glue to flow into - maybe sanding or roughing it up would do the same.

For the kits I made up, I started cutting the groove (just with a dremel), but also switched to printing all my sprockets in ABS at a higher temperature, and also printing them with a slight tooth down the hole. It took a bit of tuning but eventually got sprockets that were a tight push fit - the tooth didn't really necessarily engage with with groove cleanly, but the plastic was soft enough to conform, seemed to grip ok and had the benefit that they could be levered off again afterwards (well, with a bit of huffing and puffing anyhow).

For the new machines, the motor shafts have a machined flat on them, and the new sprockets have a proper shaped flat in them, and the motors run a lot cooler too, so its much less likely to cause problems.

Good luck!
sn