Originally Posted by jpescatore

This may sound gross but: with the nut fully loosened on the presta valve, you should be able to blow air into the tire with your mouth. If that works, not the valve.

The other posts asked the right questions, one addtional one: the pump you are using can do presta valves, yes? And you have the inflation head (what you are pushing on the valve) in the presta setting if it can do both presta and schrader and isn't a dual use head?

Originally Posted by blakcloud

Is the valve long enough that it sticks out of the rim far enough? Valves come in different lengths and if it is too short the pump won't grab it. If this isn't the issue, take the tube out and try inflating it and see what happens.

Originally Posted by Tamiya

Before I bother putting tube inside tyre, I usually inflate it first... and leave overnight or longer.
Then deflate & install.

saves a lot of rework in long run

Originally Posted by freeranger

In addition to what others have said, think about how a presta valve functions. There is no spring (like a shrader) to bring the valve back up to seal. It relies on the pressure in the tube. So when the tube is completely deflated, there is no pressure pushing the core back up to sealed position. You may need to try inflating a little by mouth, or maybe have the valve at 12:00 position, so the core falls closed after a pump stroke.

Originally Posted by AndrewJB

Make sure the valve core is tight in the stem. I just had this happen on a wheel, the valve core got slightly loose, and it would not hold air.
But wait, I just noticed OP says he can't seat the pump on the stem, that indicates he might be using the Schrader setting on the pump, as noted above.