only download every other episode

[whitelodge]

I add a series to Sonarr and then search for all available episodes. It scans to automatically add episodes but only adds the odd episodes. Sometimes only the even episodes. If I search all of the episodes again it adds about half of the missing ones. If I search again, it adds more episodes until there is only one left. I usually add it manually. Even if a series only has two episodes it only downloads one and then I have to go back and download the other.

It is annoying but I've ignored the problem thinking I would work on it when there was time until today. I decided to use NZB360pro on my Android tablet and had exactly the same problem. Both browsers are using SABnzbd installed on a Windows10 machine upstairs on the LAN. Obviously the indexer and the newsnet server are supplying all the files. Both of the browsers are downloading all the files (eventually). So it seems to be a problem with SABnzbd. It fails so methodically it has to be a setting somewhere that I've messed up but I can't find it. I've done all the standard simple things like clearing the cache powering down and bringing computers back up and even the entire network. I have installed the latest update and it hasn't helped. I don't have any idea where to begin troubleshooting. Can somebody help me please?

[safihre]

Sonarr downloads the file locally before adding it to Sab, so it's not a fetching problem.
Have you checked Sonarr logging if it shows failure?