Are you pulling the head away and rinsing down properly afterwards? I always go over twice on a new wfp clean. I think Peter told me the water should be alright to use even up to 010, correct me if I'm wrong! So if your rinsing properly, & your metres working correctly, you could be catching dirt pockets in the side of the frame which Ken mentioned. Failing that, open up your pan sprays a little with a touch more pressure & longer rinse.
Consider biting the bullet, going back to the guy that sold you the system & show him the results you are getting. I'm sure he'd oblige.
Hope this helps.