Quick update. After deciding to try it again, I seem to have got this working even though i haven't changed anything from the last time. It seems to be a lot more consistant in logging in now.
I do still have a couple issues though, that i could get some help with:
1. If you choose not to sign in when the Google Play pop-up appears OR you sign out after you've signed in, i can't sign back in to Google Play services unless i exit the game and go back in and wait for the pop up. I have a button setup to login to Google Play if they aren't logged in already, this doesn't work as everytime i press it i get 'login failed' (This is custom message i put if it fails to login to Google Play). If i choose to sign in at the start when the pop-up appears, that button works as it's used to show leaderboard if the user is logged in, so it's only not working as it should when the user isn't signed in. (I hope this all made sense)
2. There is a delay in submitting scores to the online leaderboard. I have a button where you submit your score after its pressed, and a message will come up telling you that your score is submitted. Whenever i do this, this is a delay of a few seconds before the message shows up and the score is submitted.
Has anyone else encountered these issues and fixed them?