I agree with Steve, Cabbage root maggots would be my prime suspect
based on your description. Difficult to control. There are several
beneficial organisms on the market that are rated as effective
biological controls Gnat Not, No Flea, and Steinernema carpocapsae.
Difficult to find in small quantities. Chloropyrifos(Lorsban, Nufos)
and Diazinon are effective, but not sold to the general public. You
need an applicators permit to buy them and again, not sold in small
quantities.

Jim. Cauliflower does best at a soil pH of 6 -7 . Limestone or hydrated
lime is used to raise the pH of an acid soil. A soil test ( see your ag
extension agent) every few years is highly recommended. Most brassicas
are finicky in acid soils as are beets and lima beans.