Its definitely not the thumbnails thats causing this
Honestly, I think with music, people have to be in the mood for the style... For example some days i'll listen to Coldplay all day and thats it, nothing else... Other days i'll listen to every genre under the sun in the span of a few hours... So I think maybe this could be some what of an issue for you? Not everyone will feel like listening to the style of music you upload every day, and if they do, they will probably already have artists and lists of songs they listen to, and rarely stray away from it...
SO I guess, potentially the best way to fix your issue is to get one BIG song out there (I know 10000% easier said then done)... Something that people will start to binge listen to (and share hopefully), then add to their playlists, which may lead them into listening to your other songs, and subscribing and eventually being a lot more active!
I know it's going to be a long process, and I know this answer probably isn't the greatest in the world, but I hope it helps in one way or another, at the very least it gives you some insight to how a viewer from the outside sees it
