Check to make sure the water valve gauge pressure is still adequate.
Make sure the filter is clean (on cartridge filters the cartridges may need to be replaced).
Check to make sure one or more nozzles (excluding PCC2000 fixed nozzles) are not staying up (this could be a dirty or bad Module in the water valve).
Make sure no one has removed nozzles and put them back in a different body than they came out of.
Make sure no one has opened or changed the positions of valves at the equipment pad sending water to some other feature during the cleaning cycle.