Mould problems with sunflower microgreens. Help!

Every other tray of sunflower microgreens gets white fuzzy mould by day 4 or 5. I am soaking seeds for 8 hours, sowing on coco coir, and keeping them under a humidity dome for the first 3 days. Room temperature is about 21C. What am I doing wrong? My other varieties like radish and broccoli are fine. It is only the sunflowers.