An Upper Spray Arm no longer revolves

The spray arm which resides underneath the upper rack has stopped revolving. I was able to start the dishwasher cycle with the front door open to confirm that water is coming out of all the holes in the spray arm and with sufficient pressure; the arm simply stays in place. Any ideas? Is this an easy DIY fix? Would very much prefer to avoid the $150 service call, especially if it’s a quick repair.

Thanks in advance.