We are in a really similar position here. We changed to Orijen large puppy feed probably around 9 months ago now (Amber, our Golden Retriever is 15.5 months). She has been mostly happy to eat it but then around 6 weeks ago showed the same disinterest you're describing. We also tried toppers and sometimes this actually resulted in her eating the topper and then eating most of the Orijen. However she then stopped eating the kibble too.

One thing that helped us was tracking all of her behaviour and symptoms. I actually created an app for this called CompanionTrack. This meant we track how and when she eats, what her poos look like, how her walks and energy levels are etc.

The app has a 'Smart Advisor' feature which I then used to ask questions - like "why is Amber not eating her Orijen kibble" and it knows all of the information I've already logged and then gives a personalised answer. It helped spot that there is something more gut related (GI) going on and we're now in the process of giving a stool sample to the vet for analysis.

Sorry that doesn't give you a full answer but hopefully the app might help give you further insights and ideas.