the solenoid is new as of Feb. it shouldn't be worn out. I will try some oil on the sides. the spring seems fine to me. I guess if it's not aligned right it could stick on the way back up. The rubber seal at the head of the piston could rub and stick. I will try that and see if it helps.
I also find that when I turn on the power, I don't always get the clicking sound I was used to hearing which is the piston being pulled down by the magnet.
I will try a few things tonight and see how it goes.