If you can, turn "off" half of the VAN nozzles and see what the others do.
I've seen a whole zone of VANs @ 180* each where they all leaked some around the seals (10 yrs old). They all sprayed but not well. After replacing them all, pressure and performance was just fine.
Could also be that the valve for that particular zone is faulty and not opening all the way. Could also be a partial blockage in the zone.