I would start out slowly at first and just pick up one foot and then drop it again keep doing that but hold for a few seconds longer each time . Work each foot in the same manner. If the problem is that she won't lift her foot, just keep tapping it's foot by its fetlock until he moves his foot . It could take a few days of this off and on and then it should get easier. Also when they do give you thier foot praise the heck out of them. Or give a treat if you like. I do both one day a treat the other day just praise. That way they don't get used to having a treat all the time.
Before you worry about re-training, you might want to take him to a vet and make sure that there is not some physical reason that the horse doesn't want his feet picked up. If it hurts him to stand on 3 legs for any length of time that will make him resistant to giving his foot. If he is sound, then worry about retraining. What I do with a stubborn horse is hook the hoof pick under his fetlock and apply pressure until he picks up his feet. Once his foot is in your hand, don't let it go until he stands quiet. Only hold it for a couple of seconds (so long as he stays still) to start with and then start holding them for more time until he is willing to hold them up however long you want. :) good luck