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.
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