I'm afraid matters have gone from bad to worse.
I open up the actuator. It has the small internal diaphragm. I didn't see anything wrong with it, so I just cleaned it up, and put everything back together. I also ran some water through it before putting it back together.
Now it works when I open the bleed screw, but when activated electrically, there is only a small trickle of water coming out of the heads.
Just for grins, I slipped the solenoid coil off of an adjacent actuator, and put it on the bad valve. Same results, just a trickle.
When I had it apart, I didn't see any kind of mechanical connection from the solenoid to the bottom of the actuator, just a hole, so I don't even know how that works.
I don't want to invest in a diaphragm because I don't think that's the problem, so unless you have another suggestion, I think I've going to have to by an new actuator.