I think on Presta valves, after you unscrew the little dohicky and it is all the way at the end of the piin, you are SUPPOSED to press it in a bit to let a LITTLE air out before attaching the pump to it. I was told this "opens" the Presta valve and makes it unstick and ready to accept the pump valve. Note: it can be a bit startling the first time when the air shoots out on your finger, but after that you expect it.