I have an issue where some files (usually of only a few days old) just won't download.
In this case Vipernews has 100% completion of the articles tried but it won't try any other articles and I have no clue why. NZBGet does not cancel or continue, it just sits there for days on end waiting. It will also not try any server on a higher level than 0.
I also often encounter an issue where it can't download the very last RAR from the NZB and it just sits there for days waiting, not even downloading all the PAR2 files. When I manually delete the RAR and force the download of the PAR2 files it often can repair the file.
Ok so in this single instance NZBGet could not connect to Supernews, a level 0 provider which meant it just sat there trying until infinity. When I disabled that provider it tried all the others and the availability shot down to like 1% on all providers.