Please provide a capx example to get better help.
I'm using this feature as well and it's working fine on my end.
Have you setup the google play games service completely?
My google leaderboard are publish, the beta testers has acess
in the c2 the leaderboard ID is correct,I followed the steps of several tutorials including this: "https://shatter-box.com/knowledgebase/cocoonio/#cocoongooglegamesgames" but it still did not work