I have successfully added the google play services, login works and achievements. But leaderboards just display a swirling circle of doom at me and it never loads, although on very rare occasions it loads. These connection issues lead to me not being able to submit scores. Anyone else had troubles with leaderboards?
I solved my issue which was pretty similar to yours. I hope this solve your problem as well. Here is how I find my issue:
logcatto see all errors related to Leaderboard where I found that I used wrong Leaderboard ID! Interestingly enough Unity returned success even with wrong Leaderboard ID, that’s why I didn’t expected this initially!
$ adb logcat | grep LeaderboardAgent
The second time I had your issue that Leaderboard UI just not loading and in my cause even caused my game to crash! So I uploaded my latest APK to test in on a different device in Testers group, where I found out that I used different Keystore for that build by mistake! So I change my build settings and used same Keystore I uploaded initially and everything worked as expected.