Happens. Usually from just pulling the pump off a zillion times. Eventually the stem/tube join gives out. Some brands of tubes/runs of tubes are worse than others. Some pumps grip tighter than others. You don't need the nut at all. I only use them occasionally with aero rims when the valve stem clicks on the rim. The nut is probably most of your problem. I've had friends rip out my valves "helping" me to get my tires pumped with a hand pump on the road. Great care there, or use a hand pump with a hose.