You may be too close and applying too much flame. I have found that the high temperatures cause all the volatiles in the tinder to burn off very quickly. Try holding the lighter slightly farther away and keeping it to a 1-2 second burst.
That is more or less the case. As to the science of it, I will let someone with a better education speak on that.
In practice though, I have learned to keep the flame out of contact with the tinder. I ignite the flame and then move it toward the tinder only until it lights. Works every time for me, but YMMV.