Just to make things a little more complicated for you:
Environmental allergies are more common than food allergies, so it is possible that the improvement you're seeing is due to the change in weather/seasonal allergens.
But like others said....different dog foods suit different dogs. When I had two at the same time, I actually bought two different dog foods.
And again quoting from above, I usually look for lots of energy, good poop, shiny hair, clear skin...so long as they eat fairly willingly, appetite doesn't concern me that much.
The last thing I want to get involved in is having to add little treats to get a dog to eat his food. What you have is what you get, puppers.