1. Ignition push button switch shorting out internally allowing 12v to remain on solenoid. Pull 12v wire to solenoid with it energized (improperly) and see if it releases....if so, switch bad. If not see 2.
2. Linkage sticking. Disconnect linkage from solenoid and see if it moves freely...by hand. If so, go to 3. If not, clean and lube as necessary and put back together....should work.
3. Solenoid sticking. With linkage still disconnected from solenoid, push the button energizing the solenoid and release. If the solenoid remains engaged, sticking solenoid.