grindal worms not crawling on top of food

[Ben Rhau]

Hi, all. Ever since I got grain mites in my grindal worm culture, the worms continue to produce, but they only eat food from the bottom if I lay it on the substrate. They don't crawl across the top of the food, which means I can't harvest them by resting a piece of plastic on top. Anyone else experience this? Does this mean I need to reculture the worms? Thanks