the upper spray arm dose not spin?

i bought a beko d3422fw dishwasher and the upper spray arm for some reason dose not spin there is water comming out of the jet and it dose sping when you push it round i have taken it appart and cleand and descaled every thing but still it only moves very very slowley takes about 30seconds to rotate once, any ideas on how to fix this problem{the bottom arm spins fast and properley. many thanks