Hi jb,
While I'm just diving into "really" understanding my system, I accidientally found out you can pop off the top of the 5000 head with a tweaker screwdriver and look. It seems to function like a dirt keep-out-cap. Anyway, I haven't had your issue, but when I moved into my place 2 years ago, I had to play with them a lot to fix the useless installation that watered as much patio, sidewalk, and road as grass. If you cant figure out the screw (I'm wondering if it backed out too far?) I wonder if you can re-rotate a short amount and test?