We've used Kongs with Lyra since she was a puppy. At first it took her a couple of days to "figure" them out, but now she LOOOOVES them. We have a normal, small-sized Kong for her, and anytime she sees it she automatically runs to her kennel because that's normally where we give it to her. We usually just fill it up with some peanut butter.
We also have a Kong Wobbler for her, which dispenses treats/food if she knocks it over correctly. This also took her a while to figure out, but now we use it to give her part of her breakfast. She's pretty good at getting the food out.
I'd say that at the end of the day, it does depend on the dog. Maybe you could try some different stuffers? Peanut butter, cheese, apples, etc?