Sounds bad, sorry. Perhaps some info at the link will be able to help you. Keep up the slug bait for sure, but perhaps an additional treatment or approach is in order. Slugs usually work from the margins of the leaves here (if they don't get the whole sprout.) I can't imagine them leaving a lacy pattern.

Please report back what you find. Maybe a county extension office can help you if you take in a sample leaf?

Here is a link that might be useful: pests