I'm sure other, more experienced people will chime in, but my initial thought are:
1.) Wiring issue. Not likely, but I suppose it's possible there's a problem with the harness somewhere.
2.) Dirty (contaminated) fluid causing the valve to stick. Not a problem with the solenoid.
When you had the side cover off, did you happen to notice any black powder or goo sitting in the bottom of the case? Maybe looked a little like this:
