All the suggestions so far are "damn fine suspects". But don't forget an oft overlooked issue with steam locos, especially with very old models it seems: that of "Driver Quartering".

The drive rod crank pin on one drive wheel of each axle must be exactly one quarter turn behind the crank pin on the other wheel - all drive axles must have the exact same angle. Otherwise, the drive rods will bind up the drivers. I really don't understand how they could get out of adjustment, but it seems to happen. Perhaps when one of your young sons slides the loco down the track, as I caught one of mine doing once.

It's a tough adjustment to make without a "Quartering Jig". But MicroMark may still have them. That's where I got mine some years ago.

Good luck !

OPB