what's wrong with my SMEG ST991 dishwasher?

The lower arm of my SMEG ST991 dishwasher spins, ejects water and cleans the dishes in the lower tray well. However, the upper arm does not spin, despite water coming out of its jets and having no impediments to its rotation (I've also checked that the arm is clean internally).
What could cause this? The motor must work but is there something after the motor that powers the water to the 2 different dishwasher arms?