Bosch Exxcel dishwasher spay arms

I have a wierd problem with this in that sometimes the top arm is working and the bottom isn’t and at other times the bottom is but the top isn’t ?.

I can see this by quickly opening the door when its running.

Both arms are totally clean etc.

I see in the bottom of the machine there are two separarte “ports” that feed each arm separately so I’m guessing there is a valve or something down there that is not working properly or getting stuck ?

Help !