It sounds like the dreaded cabbage root fly. For some reason I rarely have many here but other gardeners who suffer them lay a round or square of card on the soil around the stem of each plant (about 8-12" diameter) which serves as a barrier to stop eggs laid by flies from entering the soil. It is too late in your case, usually a barrier is laid at planting, sorry!
Sometimes they are present, but scarcely evident because damp conditions allow new roots to form. Not this year, in the dry weather, when plants need every root to be working and struggle to make new ones – and after July we may expect similar problems from lettuce root aphids. Their only remedy I know of is water.