I'd hazard a guess that you were right about the plunger hole.
The 24 is almost exactly like the 219's and 220's only the lever actually cocks the gun as it's opening the locking bolt on the single shot hammerless models. With the 24 it should actually be easier as it doesn't have to cock the guns firing pin as at the same time. So basically all the plunger does is force the locking bolt downward to break open the action.
Does the spring seem real strong? I've had problems like that with main springs but never a plunger spring. I'm surprised you can't see whats binding if the butt stock is off you should be able to see all the internals doing their thing in there.
