When a player is playing the game, they are executing lots of repetitive cycles that form the core mechanic or inner loop. The outer loop is just the cycle of the player coming back to the game again and again.
A player can have a lot of fun playing a game, but when they stop playing they may not come back, or may only come back when something reminds them, "hey that game was fun, I should play that again."
I'm looking for improving that part of the game-- reasons to come back and start another session. Things like having goals long term goals and progress, avoiding progress loss, social elements etc. Trying to get a higher position on the leader board would be an example.