Bottom rotor not turning, pans on bottom shelf not clean?

I have a Bosch dishwasher SGV43A03GB about 5 years old. It is no longer cleaning pans on the bottom shelf. I have meticulously cleaned the rotors, the connecting ductwork, the sump filter, removed and completely cleaned the plastic internal assemblies (matrix ?), the bottom pipe, the drain etc. After a cycle the sump is almost empty and clean. Things on the top shelf are perfectly clean. Whilst opening the machine during the cycle the top rotor is turning and spraying water but the bottom one is not. So - should both rotors turn together on this machine ? What is the possible problem ? Are there any other tests I can perform ? In the past I have successfully cured cleaning problems just by removing gunge from the internals, however in this case I am happy everything is clean. Advice please !