Engine and prop run fine at low RPMs. It pushes our 8 foot inflatable Zodiak just fine.but...when I rev it up the engine RPMs increase but the prop doesn't. I've disassembled the lower end and checked gears and drive shaft. Nothing seems amiss. What am I missing here?

the prop hub is spun

The shear pin is fine. Are you saying the hub can spin on the inner housing even when the shear pin is properly seated in the housing?

The rubber hub can slip inside the prop