Couple of things you might want to try as I recently used the Advanced Sessions plugins with no issues:
- Are you using the default game/app ID? If yes then you might be getting to many results and you should apply a filter[1]. This will make sure the results returned are for your game. (The picture is from an older version of the plugin but still applies)
- Reduce the amount of Public and Private connections to about 5. If on the test app ID Steam could be limiting connections.
- Make sure you are logged into Steam before starting the game
- If you are testing in editor, make sure in the Play Modes you are using “Selected Viewport” as this better simulates standalone and gives you the ability to run multiple instances of the game on one PC. If it works like this on one PC then move to a packaged build and test that.
[1]
Let me know the outcome of all these changes and I’ll try to help if I can.