Upper spray arm not turning

My dishwasher has suddenly stopped washing properly and the problem seems to be that the upper spray arm isn’t turning. I’ve taken it off and washed it and also cleaned all filters at the bottom. The problem could be the feed pipe at the back but there doesn’t seem to be any way of checking it (tried poking a knitting needle in…). As the lower spray arm is working, I assume it’s not the pump. Any ideas?