I just wrote an article for the forum library on growing and blooming the phal. I go into some detail about light, temperature, etc.. requirements for them. Check out the article here: PHAL ARTICLE
Right off, my first guess is always the light issue, but you say you are growing it in the proper light, so my next best guess is going to be food. It may not be getting the nutrients it needs to be able to bloom.
I am sure several other members will have something to add.