I did exactly the same, Mabel is a kong addict, she even gets excited if she sees a Kong in the pet shop. (They had a very strange low level display!). She has 3 original shaped Kongs, 1 goodie bone, 2 wishbone shapes and one shaped like a dumb bell. Her evening meal of kibble is frozen in 5 of them. They do appear to be expensive but they are by far the most used of all her chew toys. I plug the hole with cheese and have some cups that fit the kong perfectly so I can freeze them upright. When I take them out of the cup the cheese stays stuck to the bottom.
Our Bella also had no interest in the Kong when first introduced to it. I had to find things that she would do anything for and over the space of weeks and months I've transitioned to just soaked kibble. What you could try is adding some fish oil to the soaked kibble which will give it a stronger scent and perhaps make it more interesting. You also don't have to only add soaked kibble, you can add layers of soaked and non soaked. This may allow her to learn that the kong means yummy treats will fall out until the point that she learns to lick the solids out.
We are trying Kongs with Eric and unless it's something that comes out easy he's not interested After our first few ignored Kongs we filled one with dry kibble so that he could get food out easy and that got him interested. Banana and cottage cheese have been a bit of a success. he gives in really easy, barks and looks at us as if to say 'come on... get it out for me' haha. I'm trying grated carrot today and soaked kibble I haven't frozen any yet
I have to help out sometimes. Using a plug of peanut butter on top of slightly soaked kibble and then frozen works well. By the time the easy to get plug has been eaten the rest is beginning to thaw and makes it easy. I have to jab with a knife on occasion. Make sure you have the right kong for the teeth your puppy has too. Sometimes I use a plug of live natural yoghurt.
