If your netting isn't secured properly all round, birds may get in and they will strip brassicas. If there is nowhere obvious that birds could have got in then it's most likely cabbage white caterpillars. If you are using normal open mesh netting, it won't keep the butterflies out. You need close weave protection like scaffold netting or enviromesh tightly secured all round. If you install something like that and get rid of the existing caterpillars, you may be able to save your crop. You can try searching for the caterpillars and squishing them but you're unlikely to get them all. If you don't mind spraying, use one of the commonly available bug sprays.