Gentlemen;

The third possibility, is that the pilot envolved didn't use a spacer to properly seat /centre the prop on the shaft.

Keep in mind that this is the 'Beginer's Forum' and missing such basics is common place.

Most engines come with a series of 'Spacers' of different thicknesses, these slide onto the Prop shaft to ensure a snug fit with the prop. Propeller manufacturers have different diameter centres - Engine manufacturers have different diameter shafts and compensate with "SPACERS"

IF you do not use these spacers the prop is not centred on the shaft and you will have the biggest out of balance propeller condition imagineable.[:@]

Before you spend any money, look for the free spacers - I hope that you didn't throw them out saying "What the He!! are these?"