As mentioned you need to keep them greased good so they aren't binding.

There are two places that wear that can cause the clutch to not seperate far enough to disengage.    The first thing is the clutch halves where they turn and push on each other.   If they are worn you can either replace them or build them up with the welder.

The other place is the strap that holds the clutch from rotating with the shaft.    The hole in that strap that attaches to the lift wheel assembly gets worn and won't rotate the clutch enough to disengage.      Build it up with the welder or get new.