I know some business fares can book into Y buckets when there is no business availability on a specific flight. I assume that similarly first fares could book into a J bucket for the same reason.

It’s a bit difficult to give a better answer without having specific dates/flights but what I assume is that you’re trying to book a non flexible business class fare and there’s no availability on the flight in any non flexible bucket.

Are you looking very far in advance by any chance? I think flexible buckets tend to get released before non flexible ones.