By scrolling through your leagues' leaderboard you may see a PROMOTION ZONE on the top and a DEMOTION ZONE on the bottom. Make sure you make enough score during the league period, so that you are above the PROMOTION ZONE marker, to be promoted to a higher league. If on the contrary, you did a poor score during the league period, and you are below the DEMOTION ZONE marker, then you will be demoted to a lower league