If you're "popping" a tire and there's no penetration, then it's one of two things:
1. the tube is getting pinched between bead and rim, or
2. the bead on your tire is broken.
You're pushing the valve stem up into the tire, but are you going around the tire on both sides checking for the tube being under the bead?
OK,I'll add a third possibility. If you're running low pressure, the tube can slip inside the rim, and the valve stem can get ripped off. I think you would have mentioned that if it were happening though.