Hello, first we chat a bit, then I give you some partial answers, please do not click accept until you get a final and complete answer you like.
If the motor rotates the coupler, but the attached shaft does not turn, the coupler set screw is loose, look at that first. tighten it an retry. Get back to me.
Hello again.. from here I am totally blind, I only know what you tell me about whats wrong... so tell me :
Is the coupler connected to the drive motor shaft, and is that shaft turning? thats what I understood from your original post.
Let me know what you are seeing on your end, we can go from there.
Hello again, these things are not built to be repairable at that level... but Im sure with your demonstrated ability to dig into it you can eventually repair it. Normally such items are just replaced. One finds on diggning to such things that often the parts are not available even though listed on the mfgrs legally required parts list, and the time required to repair pales in comparison to the time and costs of just buying a new drive.
let me know if that makes any sense on your end...we can go from there.