I was worried when I started reading the comments but after watching a youtube video I learned you need to gently unscrew the bolt on the top of the presta valve. Then apply the pump, and screw the pump onto the valve and start pumping. If you dont unscrew the the presta valve bolt, it will feel locked and air flow wont get through. Should work like a charm. Good luck.

I am surprised by the negative reviews. I have been using this pump for over a year and a half (I ride daily, about 9,000 miles per year), and have not had any problems with it. I often help inexperienced riders, so I've used the pump often on other people's flats. Things I like: 1) it is compact and light. Yes, that means that it is low-volume when pumping, but that is an acceptable trade-off for light weight. 2) The head is dual (Schraeder and Presta) 3) The head threads on to the valve, which makes for a secure attachment with very little air loss during removal.. 4) The head is at the end of a short flexible hose, which makes inflation much, much easier.