This is an electric clutch and the engine cooling exits the front of the tractor, so I suspect the heat is causing the internal components of the clutch to expand and thus not engage.
Take the front grill off and try using some point lubrication (with the little red tube) using wd 40, then some white spray on lithium grease to free this area.
I have a cub 1990 1811 hydro w/400 hrs no problems with the clutch to date..
Carl