Does your spinner have a plastic backplate? If so, these don't provide enough friction and the backplate will slip against either the prop or the prop driver which loosens the nut. Friction can be increased by making a washer out of a piece of wet/dry paper or very thin leather from gardening gloves. The best way though is to use an aluminium spinner. With enough friction between all the parts it's surprising how little the prop nut needs to be tightened. When the engine is running, the power impulses when it fires tries to tighten the nut.