also, try a bunch of different things. One of my dogs loves the Kong, though has trouble getting to the end of it. My other dog is fussy. So far the only time he's finished the whole kong, is if I put a big treat in there that he has to work a bit to pull out, sour cream, and strangly enough, his normal kibble. He doesn't get excited at all by his kibble unless I make it a bit of a challenge for him to get...
As well as peanut butter you could try melted cheese, egg (I've heard of people cracking an egg in it, and chucking it the microwave for a minute), natural yoghurt, meat. If it's a hot day, most dogs will enjoy a frozen peanut butter Kong (or maybe even an ice filled Kong)