Some dogs can take longer than three weeks. Were their stools good during the transition? I usually will suggest not increasing the percentage of new foods until stools are normal. Natural Balance grain free is very low in protein and Merrick is high so that was a big change. Sometimes transitioning to something “in the middle” is a good place to start before moving on to a higher protein food. That would be a food in the 3.5-4 star range. You can go back and add some NB to their food until their stools become normal and that will bring the protein down some (closer to mid-range). As an example of a dog needing a long transition time, one of my dogs took two or three months. You can also keep “mixing” foods together just to add variety and to get the level of protein that you want.
I foster and have many dogs. I did get some Merrick grain free of them once and put too much of it in their food at first and 4 or 5 out of ten maybe had GI issues. I just cut the Merrick some more with a lower protein food and they finished the bag without any issues and I’ve fed it again since then (mixed) and no one had issues.
My personal dogs have tried a dozen kibbles and have worked up to eating raw foods. You can just give it more time or try another food as there are many to chose from.
