If there’s a way for trans oil (90wt) to get on the solenoid internals it’s a possibility it could make it sticky especially if cold. The outer coil, if good, will continue to try to move the armature but the oil if there is any will make it stick. If you can take it off and clean it you will see any oil/dirt residue. Good luck. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

This issue was discussed a couple years ago.  Rather than disassemble/clean, I just installed my backup unit (and a new trans. seal).  Now the backup is doing it.
I just wonder, what would happen if I stuck the spray nossel of a can of electronic cleaner into the solenoid drain hole (and sealed around the tube), filled the solenoid, let it soak a while, and then drain (?).

6 VOLTS/POS. GRD. NW INDIANA