If you got the right belts and renewed / cleaned all the Idler Rubbers even the small ones inside the mechanism as chartz mentioned there is a mistake which I made for myself, I took just only one cassette for testing which did’t work, took another and everything was fine.
Are the transport wheels which grab into the cassette running free? Common problem is sized lubrication on the spindles.
Have a closer look for the breakarms, one grabs inside the transport wheel, easy to mount wrong 😉 don’t ask me why I know this.
If all is done and still no progress you could change one wheel after the other from your working unit to determine which part is causing the failure.