I have 3 Orbit shrub sprinkler heads that are attached to 3 foot risers behind shrubs in the front yard. They all work great for a few runs and then all of a sudden I have a geyser coming out of one heads and the other two are still running fine. One head that keeps coming apart where the interior of the head flies out into the yard. The part of the head that screws onto the nipple is still attached, though. I've replaced the head 3 times with brand new heads and make sure that I am not adjusting it so much that the interior is loosely fitting. What could be causing this problem? Thanks for your help!!