Hi Eileen,
I remember having some difficulty getting my BC inflated orally at the surface when I first tried it. Make sure of the following:
(1) You are getting a good seal between your mouth and the inflation mouthpiece (i.e., the air you're blowing in is not leaking back out)
(2) Pay close attention to the timing of when you blow and when you push the valve button. If you push the button too early, you allow air to escape. If you push it too late, you've wasted some of your breath. I breathe in deeply, put my mouth on the inflator and then push the button just after I start blowing (no way for air to escape). Since you're in the water while you do this, if you push the button too early or hold it down for too long after you've stopped breathing, the water pressure forces the air out very quickly.
Hope this helps -- stick with it and you'll master it with a little practice!