Selgas
Have you checked the rotating arm to ensure that all the water oultet holes are clear and not blocked up – this is a very common cause to exactly the problem you have highlighted.
If they are blocked them simply clear them out – this allows the water being sprayed against the dishes to do it job properly.
Another cause of this problem if the spray arms are not blocked is a blocked water filter feeding the pump – this is a job for a service person not a home owner. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
