You might have cabbage worms eating their fill of your tasty brassicas.

FOUR EFFECTIVE WAYS TO DETER CABBAGE WORMS NATURALLY:
Handpick them off daily.
Try companion planting—interplant with dill, thyme, and rosemary.
Put floating row covers over your crops before the cabbage moths lay their eggs.
Apply Bt, Bacillus thuringiensis (Bt), an organic, natural soil bacteria that targets caterpillars safely.