There are two possibilities that come to mind. The prop has a neropreme
insert between the splined coupling and the propeller to absorb shock. They
sometimes wear out and cause slippage as you mention. In which case a prop
shop can install a replacement insert. The other and less likely is the
splined coupling on the crankshaft of the engine. The drive shaft to the
lower unit mates with this coupler. It is a neropreme filled cup that also
absorbs shock. This requires pulling the engine to replace the hub and
require a hoist to lift the engine. Hope this helps.

Don Dando