With peanut butter, I've found that sometimes they don't get every single little drop, but that's okay with me since I clean the kongs out after they're chewed on anyways. I like to take a little peanut butter, some unsweetened apple sauce, and some cinnamon, mix it together, put it in the kongs, and then close it with yogurt, and freeze it. It takes them a while to get it all out, and it's healthy.
EDIT: Lol, just realized I've got one of my Kong Dogs in my siggie