Have you tried mixing the puppy in with some junior, and then over the next few weeks add more junior than puppy... Until she is only eating junior food?
My baby is a bit of a fussy eater, but I haven't had to change her food as she is just 7 months old and on Royal Canin, plus chicken, cheese, tuna, salmon or anything else I think of adding to it!
If it was one of my Newfs I would just tell them to get on with it or they won't get fed, but with my little Cocker I think I would be stressing about her starving to death because she is so tiny to me... She is 8 1/2 kg though so that is probably right for her size!