I'm in the same boat. I think it has to do with how many views, upvotes and comments you have in a set period of time.
Just persevere. By the numbers, you're on the right track. Look at the memes that did well. It's disappointing when you don't get there, but one day, along the way, you'll make it. Right now, my goal is to close in and make the leaderboard. I've only had on meme that made it to front page, several that were at or near the top of page two, a ton on page three. Just keep a positive attitude. I started from scratch in November, and I'm closing in on my goal.