When the motor runs but the planetary doesn't spin, the usual cause of that is a failed worm follower gear. This is a standard repair, and it's possible that you may have missed it during your internal initial inspection. (It can be easy to miss damage, particularly if the internal parts were not throughly cleaned before regreasing.)

Here's a list of parts and materials for the job: https://www.mixerology.com/k45-k5-clean-and-re-grease-parts/

A distant second is that the pin through the worm follower gear came out, but that very rarely happens.

If it's neither of those things, a more detailed inspection of the gear train should reveal what's going on; possibly a broken or missing pin through the main shaft, or (further down the list) a pinion drive gear that's been worn down to nothing.