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If the head is installed so the top is not above grade, these things can happen, and you are basically out of luck, since the mfr instructions were not followed.
The top of the cap and the top of the riser must be above grade, which is counter-intuitive, since it will look like a tripping hazard. It's the only way that the riser part operates free from the presence of dirt.
Every carton of rotor heads from the major manufacturers will have a diagram showing how the head should be set. And it's always a bit higher than you'd expect.