As stated above, clean them out. I always thread the new ones in a couple turns, then back them out, and repeat by going another turn each time and then backing out. Eventually, they'll go all the way in fairly easy. It's also quite easy to cross thread those even though you'd swear they were threaded properly. Often, just to make sure, I'll slide the hyfax back out of the way to visibly see the hole and thread it in to check for accuracy; plus it gives me a reference as to the angle of the screw.
(I have many of those screws [new] on hand just for such an occasion).
Jon