Something that they enthusiastically ate when you 1st opened the bag, but after a few months, they stop eating it...it could be bc the fat &/or oils in the kibble got rancid.
Freezing Dry Dog Food
I have 4 mouths to feed, so a 12-14# bag is done in 2 months. I keep their kibble in the fridge in a lock tight tupperware. Have you tried moistening it in water? Mine all like it moistened & I mix some canned food for din din & they get hard kibble w/the protein of the week (this week is tilapia) crumbled on top for breakfast. Except Momo, he gets moistened kibble all the time bc he doesn't chew his kibble.
Like Deanna mentioned, try a different formula. We stick w/the grain free versions. They love all of 'em, duck, venison & now they're eating fish/herring.