Check the tightness of the screw that secures the gear on the end of the roller shaft. If that gets loose the gear and the worm will not mesh properly, and you'll chew the gear up, which makes it hard to turn. Another cause of this is wear in the worm bearings, or slop caused by loosening of riveted posts. Those can be hard to fix. On really old machines I've seen the shaft hole in the tuner plate wear enough to cause this sort of problem as well. If they're worth the effort, or use a non-standard spacing (usually 35mm between posts these days) you can sometimes disassemble the tuners and silver solder in bushings of brass tube.