Stores & services like Beatport and Apple do NOT allow songs to be added on a release after it's already shipped. This is a limitation of the stores, so there isn't anything Proton can do about it. Luckily, there is a way to get a track added if needed!
If you need to add a track to your release, then you need to cancel/takedown that release and then re-create it via the Compilation Tool. Here's our guide how to do it.
To start, create a new version of the release. You'll need to choose a new catalog number, but it can have the same title.
After creating the new release, re-upload the cover art.
Then, click "Compilation Tool" and re-add all the tracks from your original release. If the songs are not appearing yet, contact support@protonradio.com for help.
After adding all the original tracks, you can now add any forgotten tracks.
If your original release had any Release expenses, you may need to re-add them.
Schedule the new version of your release, then ask support@protonradio.com to CANCEL the original incorrect version.
If that sounds like too much work, you could also just release the forgotten track on a later compilation or a special free download on SoundCloud to boost followers.