I am not intimate with your machine, but here's what I '''think''' is happening:
The two wire stop solenoid has two internal coils: (1) a 'pull' coil and (2) a 'holding' coil. In theory, it should have a reduced power requirement to 'hold' than to unseat from fully closed position, but I truly don't know how the two coils interact electrically nor what controls them working together or sequentially.
Best bet is to find your wiring diagram and see how things are wired on the solenoid.
You could use a multimeter set to 'continuity' or 'ohms' to prove if each coil is independent and if each coil is 'good'. Test between each wire terminal to the metal hull and you should see continuity. If this is a two-coil solenoid you should see zero interaction between the two wires. If you have zero continuity between any wire and the hull you have a bad coil.
Alternatively, and with some slight risk of ruining the unit---not knowing that particular solenoid I have to mention some risk in testing as described could blow the solenoid---you could try grounding the solenoid's metal hull to the negative battery terminal and try each of the wiring terminals in turn. You may have to manually assist the 'pull' to move when energizing the 'hold' to hold open. Back to that wiring diagram.
You should be able to prove both coils are functional. The 'close' spring is also a consideration. The spring could be bad or both / either coil could be bad.
Try a tiny dab of non-conductive silicone lube on the solenoid shaft if it appears to be binding and work it manually to free it up.
One last consideration: make certain the solenoid mounting area on the machine is clean. Could be the solenoid case is poorly grounded--paint, grease, dirt. Use a jumper from the case to the negative battery terminal and see if anything changes when energizing.
Please post back with your experiences so we may all learn.