I sowed a few winter spinach in October and they have germinated and the seedings are now about 2 inches high but every morning I find 2 or three of the seedlings lying flat on the seedbed just cropped off at soil level. The tops are not being eaten, just left lying as they fall. I've tried lifting and sifting the soil under the seedlings to a depth of around 3 inches but haven't found anything. The plants are in a cold-frame with hinged glazed lids. Does anyone know what might be causing this? I've added a pic below taken a few minutes ago: the seedling on the right is one that has been cropped last night.