Alex
Try taking the nozzle out of the head by backing off the set screw and running the zone. The water pressure should pop the nozzle out. Then let the affected head run without it for a couple of minutes. If you have several heads not functioning, you'll need to rinse them one at a time due to insufficient water pressure. Sometimes a collection of finer dirt which has made it past the filter gets into the gears and stops them from functioning properly. Increasing the velocity of water by letting more water through the head should effectively rinse out the gears and proper functionality should resume. If that doesn't work, try removing the head (if it's a PGP) from the line and attach your garden hose to the bottom after removing the fitting. The hose threads and the threads on the head are both 3/4" but aren't exactly spaced the same so don't crank the head on the hose or you'll strip the threads on the head. Be sure the nozzle is out and open up your garden hose to rinse it out. Don't forget to put the nozzle back in the head.
Good luck. Tony