I recently set up Google Play Music to add new MP3s from ~/Music on my main machine. It worked like a charm at first, but now that I've also downloaded the whole library to that folder, it doesn't recognize when new songs are added. Do I have to remove and re-add the folder to the "music from this computer" section?
I use Google Play Music Manager and turn it on (or restart it) to check the folder(s) I've added for new tracks. It does so when it starts running. I think if you left it running it would eventually check for new tracks itself, but it's quicker to restart it.