The belt is not getting stuck. I have never had a belt jam.
The spools rotate around the center of the arm halves (arms rotate around the router and the spools rotate around the center of the arms).
The spools should be able to rotate smoothly when the arms are assembled but of course you cannot check that after assembly due to the motor’s gear ratio preventing them from turning.
You can check that the spools spin smoothly on each arm half before assembling the two halves though. If you place the spool on each arm halve and cannot move it smoothly using your hand, then there is a problem. In my case I found it difficult to rotate them and saw that there was a very produced ridge on the inside circumference of the spools. The fit was worse on one arm half than the other which suggests that the spool is the problem as both arm halves are identical.
In my case the ‘tight’ side is the one with the gears.
My apologies if I am not explaining this adequately. I will try to upload a video and photos showing the issue.