Some times Propellers hit things.... easily replaced!
1. Remove the nut on the propeller shaft and slide included propeller onto shaft.
Reinstall 7mm nut and tighten until it is tight against the propeller, you can see how the Propeller and the Drive Shaft's Brass Propeller Bushing, mate.. so that the Prop won't slip while under power. Use a 7mm wrench (NOT INCLUDED) or pliers to turn remove/tighten the nut.