I think what you're talking about here is the NZB file itself not being available when you clik on the link.
That's because FST deletes the NZB file after 40 days.
This is to keep forum overheads down; all ISP servers and some pay servers will not still have the articles by that time anyway.
Blueyonder offer a very good usenet service. Completion is very high almost all the time, although article retention is probably only 3 to 5 days (it was when I had the service). For a bundled service this is excellent.
Just check the post date in the NZB info near the top of the post before downloading.