Ah perfect, yes the exclamation mark indicates the track isn't supported - normally the song will play anyway as AJ Audio instructs your Jellyfin server to transcode to a playable format (with some features unavailable such as seeking). However, this mechanism currently isn't implemented for the Automatic Downloads feature. I'll ensure this is configured for version 1.6.
What audio format is most of your music in?
From the top of my head I believe this may be an issue with the buffer size but I will review and confirm
Thanks
Alex