OK so i came here to report the same issue. I believe it has something to do with NZBGet 17.0 as it only started happening when i upgraded to v17.0.
When Sonarr grabs a release, tries to download it and it fails, it removes the download from NZBGet and then tries to download it again at the next RSS sync. This was no problem in v16.x of NZBGet as Sonarr was removing the entry WITHOUT any history tracking.
In version 17, when you re-add an identically named NZB, it adds it with a status of COPY (New status as far as i can tell) and doesn’t download anything. Sonarr then sees this in the history and says it didn’t download it so it skips it. If you go to the entry in the history and choose Download Again, it will download (successfully this time if it had time to propagate) and Sonarr will pick it up no problem.
It has become quite a PITA as you are never sure if something has downloaded properly.
I noticed that when i delete an entry in NZBGet, it now gives me 4 options instead of 3 asking me what i want to do with the history and downloaded files. Perhaps Sonarr is telling it to keep the history or perhaps it’s just NZBget’s COPY status that is throwing the wrench in our spokes.
Here are some screenshots of the problem in action: