Well the plunger pushes up against the cam. I f you work the mechanism it may get some corrosion off and polish it off a bit.
Otherwise you may have to take it apart and clean the plunger and inner coil.
These mechanisms are just about trouble free.
Could be a warped plunger-coil stopping the plunger in the middle of its upward travel. Or a cracked/broken cam. If you remove the base, you can see the cam and test the plunger. At 16 VAC, the plunger should fly out of the coil.
Here's a video of a gate at 6.5 V DC (about 9 VAC peak). I did this test to show that a lower voltage will operate the gate, albeit slowly. But it saves the cam from the violent shock of the plunger and prevents the coil from overheating and warping. You can dial in your minimum voltage - acceptable speed - and lengthen the gates life with a separate transformer or try the 11 VAC accessory terminals on a classic Lionel Postwar transformer.