I left my dryosaurs with a log & ball toy, a feeder stocked full with plants and several fern patches, and after a few days their health was declining, so I checked them with the dinopedia and it turns out they were at 0% hunger. I checked the feeder and the numbers hadn't decreased.

Its a current issue in the mod with most herbivores. I recommend going into the config file to disable dino starvation.