Sounds suspicious... Is the endpin plug ebony? Or more likely maple? Black-painted cheap wooden endpin plugs are always getting distorted and kind of 'fluffy' in the way the pin moves through them, especially when there's a lot of humidity around. And if such a plug has been made too small to fit inside the metal ferrule where the thumbscrew goes, well, that's just fragile and prone to wear/bending/falling apart. So have a good look at the whole thing, with the pin out if it'll come out, and judge first if it's of lesser quality. Maple (or similar-looking tropical wood) plugs of smaller dimensions are often so soft that they'll distort as a whole, bending in the taper, and causing endpins to lock up or become cranky. Quality, as always, matters.
If the plug is of decent hardwood, rosewood or ebony or boxwood (though those are unusual - lighter wood usually means maple in endpins) and if the dimensioning is reasonable, then the next thing to consider is whether to have a little 'slack' put into the hole. Using a drill bit just a hair larger than will comfortably fit into the hole, and I mean just barely larger, not much difference at all, it may be possible to give just enough breathing room as to avoid such seizing up while also not introducing disastrous loose endpin problems. Go too big on the hole and it'll be endpin-shopping time. Even a bit of sandpaper taped to a dowel, smaller than the hole, and run through a few times with some care could do the trick. That is, if the pin comes out. If it doesn't - and lord, tell me why so many don't? - then you'll probably want to take it to a luthier to have the strings taken down and the plug removed to get it all tuned up nicely and have the swaging or rivet or whatever is holding the pin inside sawed off.
