Stress (physical or mental), ulcers (often stress related), teeth, imbalanced minerals, parasites. Lots of places to go before even changing the ration because if something else isn't right you just end up wasting money on supplements. I would agree that pre or probiotics are always going to be helpful for better digestion.
I personally like to stay away from processed feeds, but I do like the beet pulp, flax, and might mix that with a hay pellet to make a good mush when I have to supplement. Just pasture works best on my hard keepers, my 25-ish y/o mule will eat and eat and just seems to gain from the bottom up anymore but she is always better looking on pasture than hay+supplements.

It is pretty impossible to balance your feed ration unless you have your hay tested, depending on where you are the nutritional content of hay can be all over the place. Some horses will do ok on cubes or pellets but it really helps to have some long stemmed hay for slower digestion. Some horses on straight pellets can get acidosis/subclinical laminits and other issues.

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance