Are/were they "No Job"? Did you try 'cleanowned' in DFHack? Are the food barrels being moved all over the place?
I am on a Mac....in Afghanistan. Not very conducive to finding Mac versions of DFHack and downloading them in any sort of timely fashion.
Has your military claimed it?
I do not have a military established yet, nor do I know how to check if they have claimed the food.
How much food is there?
I have 20 adults currently in the population and about 300 prepared meals.
It's either a pathfinding issue or the food is somehow tasked for something other than eating.
Also, you can try to forcefully move it to somewhere else either by designating the items to be dumped or by, as it has been said, moving the stockpile (which might prove pathfinding issues if they will not touch it even if designated to be moved).
Dining halls are not required for eating.
Another solution could be to just grow plump helmets - if there's something wrong with the meals in your pile, you dwarves will clusterfck around freshly grown p.h.s like nerd trekkies around a nude ST actress' picture.
I'll give it a try redesignating the stockpile.