Are you sure it was going into neutral or was the prop hub slipping? Lots of time with a slipping hub you can idle around just fine but when you hammer down on the gas the props just spins around the rubber hub and you don't get any forward push. You can tell if this is happening by using a magic marker and marking a line across the hub and prop barrel. Take it to the lake and replicate event and then see if the lines match up, if not the hub is spun. You can have a new one pressed in at a prop shop.