Hmm...I had this problem too and I'm still not quite sure how I solved it. It's all in the pinkie action, so if it's not working try to do it as slowly as you can to see where it goes wrong. Remember, you want to push the outer corner of the card
forward then
down. If you forget the down part, the card won't lift up into your palm. Also, try straightening your other fingers slightly so as to provide more area to 'catch' the card as you palm it.
Also, don't worry too much about exposed angle (although that doesn't mean forget about it either

) as the beauty of the one-handed top palm is that it can be done under the cover of misdirection and the audience will never suspect a thing. It's like a pass in that regard, except you have one hand free to gesture with etc.
Kati
P.S. I'm not sure if you have Born to Perform but there's a decent explanation on there with several views.