If it sticks and you can't unstuck it.. replace it. They're not overly expensive. Probably once stuck and cleared , it's gonna stick again soon anyhow.

I'm supposed to flush my system annually but admit it only gets done infrequently. I have a sand filter on shallow wells. The whole system has gotten flushed, Maybe twice in past five years. Only two hunter pgp's have stuck ... one from being beat up repeatedly with a towed core aerator ... the other stuck and wouldn't unstuck for no apparent reason after 6yrs.

I back flush all the heads once out, it's a messy job trying to cup a hose end to squirt down the nozzle hole .. I get wet.

Here is a link that might be useful: Good Info ,,. http://www.jessstryker.com/