For Transmission make sure files are in the correct subfolder, Transmission doesn’t have proper category support, so subfolders are used, if the subfolder doesn’t match the category set in Sonarr’s Transmission settings it won’t track the downloads.
If Transmission puts downloads in C:\Torrents
and Sonarr has a category set as sonarr
then the files Sonarr sends Transmission need to be in C:\Torrents\sonarr
, which Sonarr builds using Transmission’s config (which it gets over the API) or set the directory explicitly (in Sonarr’s Transmission settings).
If you look at ((Trace logs)) you’ll see what Sonarr gets back from Transmission and can confirm that the issue is with the subfolder not being correct.