I didn't see this mentioned but I didn't read every single post on the thread..but I'll add in my 2 cents.
There have been a few times where Vash has simply doesn't seem all too excited about eating I'll give you the run down. But there are so many causes for something like this that it's hard to narrow down one specific thing.
1) The first time Vash ever "lost intrest" in his food was when he started getting used to my schedule. I wake up, take Vash out to walk/potty, we come back inside and I feed him breakfast, then we play until 7:30. The play time varies depending on how big of a pain in the bottom Vash wants to be on our walk, if he sniffs every flower it is 10 min and if he doesn't we usually get about 20-30 min to play. After that I put him in a safe area, get ready for work then leave.
At some point Vash decided that he would rather have a few moments extra play time than eating time, so he'll ignore his food and come running over to play.
I just left his food bowl in his safe area then left for work and it was empty by lunch.
2) The second time I noticed Vash losing intrest in his food was 4 days before he got diagnosed with an upper respiratory infection. He wouldn't eat any kibble unless I hand fed it to him, and it was simply because he was having a hard time smelling an aroma from the food.
A scoop of peanut butter seemed to get the point across to Vash that there was food in his bowl.
I honestly found this the most intresting, because he'll eat rocks off the sidewalk that have no smell but wouldn't eat his kibble.
3) Another time I noticed it was when Vash simply wanted to "play" with the stegging egg or other food dispensing toy. He would ignore his food, walk over to the egg and start pushing it with his nose.
Simply putting the food in the egg seemed to solve that problem.
I'm sure there are other reasons a dog can lose intrest in food. Just like they have high-reward treats and low reward treats the more often they have something the less valuable it is going to become. If this is the case then perhaps the best thing you could do is mix things up to keep the dog intrested. Be it adding water one day, and chicken stock the next, or putting a treat in the bowl, adding a topper, etc.
You probably don't want to do the same thing every day because then the dog will expect you to do it every day and god forbid one day you don't have the topper...but if you do something different every day, the only thing he can really expect is something a little different in his food bowl.