Pull it all the way up or even better out, and spray the posts with a good coating of a silicone based lube. I built seats for Ford for years and that is what we used when they are first assembled. Without it they were a real pain in the neck to move up or down. Also, make sure your headrest posts are straight, if not just put in a vise and straighten them right out. Lastly the headrest pins themselves(the piece the headrest sits in) have a tendency to like to slip and come up a little. This will also make it hard to move, if you notice one pin sits higher than the other just put a rag over it and tap lightly with a hammer till it is seated all the way in. Good luck, darn headrests can be frustating sometimes.