Assuming you're using a three bolt puller, and you have equal (level) tension on all three bolts, take it loose and reposition it. It could be pulling slightly off-center, causing it to bind on the key or the snout. I've seen some that take a pretty good pull to get 'em to pop free, especially if they've been together for 30-40 years. Probably has a good bit of corrosion build up as well. If you have an installer, you can always push it back in a little, squirt some PB Blaster around the snout, and try it again. Make sure your puller is straight on the crank snout - shim it with a thick washer if you need to (works for me).
Good luck - let us know how it goes.