Hi I have a slim line Bosch Dishwasher. I have noticed that the cleaning recently has been poor. After a bit of analysis I can see that the top spray arm is not rotating. In fact there is no water pressure to the arm when the machine is in cycle. Both the top and bottom arms have separate individual water supplies. The bottom arm is working correctly. The top arm has been inspected and is not blocked or cracked. Does anybody know why there is no water pressure to the top arm. I have watched the cycle with the door partially open to witness what is happening. Any suggestions greatly appreciated