One thing you might check is that the entire tip of the presta may be one of those srew off types.. I carry a pump but a buddy insisted that I use his CO2 on a ride while others waited for us.

So I used the CO2 with a screw on type adaptor. All went well but when I unscrewed the adaptor, it unscrewed the entire top of the valve stem. (1/4 inch long pc) INSTANT DEFLATION

You might check to see if you have this type.

There are different types and lengths of tubes, threaded, non threaded and different lenghts. When you get a new tube, make sure it is long enough to protrude your rim properly depending on your rim depth.