my 7mth old collie is on wainwrights dry as well. It is her 3rd type of food as we had the same problem as you. She can go a couple of days without eating it so i have restricted any treats like pigs ears etc. We also added things to make it more attractive but were told off !! by the vet who said she would eat when she was hungry and she certainly is healthy enough. she was on 2 meals per day but because she does not eat we now just give her one meal.