I just bought a Wrench Force air slim floor pump; however, the presta value does not seem to work. The head has a presta valve and a schrader value. Air is getting through the schrader valve but the presta valve is not allowing any air through at all. It seemed to work properly when I first started pumping but now it is stuck.

Any ideas?

Sounds like you need let some air out of your presta value first.

You do realize you have to let some air out of your tires presta value first(Just some of it). If that doesn't work take it back an get a new one or just buy a cheap presta valve adapter so you can use it on the schrader side.