If you are hiking hard for a long time, most of your circulatory system is supplying your large muscles, and there is little left over for digestion. As a result, you may eat a snack, and the lump will just sit there in your stomach. With that happening, your appetite won't be much.
This may not be a solution for everybody, but it works for me. I can go a long way on Gatorade. Also, I carry something really small and simple, like jelly beans. That will get me through the hard work until I can take a break. Then a small but substantial snack will be good.