That is typical with a flat bottom designed hopper since it feeds from the front, it will valley like that. They need to slope the bottom more and feed from the middle. I usually keep mine 3/4-full all the time. Kinda of pain in the butt, but it works.
I did a mod to mine and angled the back bottom to help push it forward. BUt I always push the coal forward every so often when it start getting below the 50#'s in the hopper. MIght angle it a bit more, it works, but could work better.
Here's my mod.Coal hopper modification