This is likely due to filters created to avoid fake rounds. We really need to protect ourselves and our users from cheating, otherwise the leaderboard would become unreliable, and one of the filters doesn't allow more than 15 strokes under the playing handicap. It's fundamental to introduce your correct playing handicap.
Example: Your course is a Par 54 and your correct playing handicap is 21. On the leaderboard your best round is 69 but, in fact, the best round you've played is 63 (which you've played with playing handicap 28, not the correct one).
- Par 54 + 21 (playing handicap) - 15 (strokes) = 60 which is less than your score 69 (so everything is fine according to our algorithm).
- When you've made 63 gross, you've played with playing handicap 28. So: 54 + 28 - 15 = 67 > 63 (that's why the round is not listed on the leaderboard).