Caulies are the hardest brassica to grow well in my experience, too. You definitely have a root-eating pest of some kind. I would try growing them to a larger size before planting out, then plant them deep - right up to the first leaves - maybe even an inch deeper. Also make sure you firm the soil really well after planting, then place a collar around the stem (see RHS link below.)
Most pests like acidic conditions so next Winter it might also be worth liming the area where you will be growing them.
RHS advice on cabbage root fly: