Hello,
The way the JNC membership works is it lets you read the latest volume that is currently being translated. After a Volume has been fully translated it is compiled into an ebook and sold on all major ebook retailers. After the ebook has been out for a few weeks all the parts for that volume except a preview part are put into the vault.
So for Bookworm the standard available time has passed on many of the volumes. You can refer to the expiring volumes topic for future volumes that will expire.
JNC’s business model is twofold the subscription for up to date readers, and they also have an ebook model for people who prefer a complied book vs weekly releases. You can always buy the books of Bookworm (and any other novels you like) to show your support of the work.
It is important to know that every month JNC has catchup titles where several series are selected to all be put back up for all subscribers. An Announcement post will be made in the news section of the forums.
Taking a quick look at the site's FAQ may answer almost all of your questions, if you have any others, please feel free to ask.