Blizzard provides third-party sites like RaiderIO access to both Mythic+ Leaderboards and a Mythic+ API. These two sources of Mythic+ data allow us to see all the runs that have been recorded during the week for a given realm and dungeon.
However, Blizzard's API will show a maximum of 500 runs for a given Realm and Dungeon. This will be the Top 500 runs for that realm/dungeon combination at that moment in time. Once a realm has had 500 runs recorded for a dungeon, then the minimum keystone level/time to rank for that dungeon will increase as new higher keystone runs are added to the leaderboard and lower keystone runs are "pushed off" of the leaderboard.
For high population runs, these numbers can become quite high by the end of the week, preventing runs by groups of people on that realm from being ranked in the Mythic+ Leaderboard/API, and therefore not be picked up by RaiderIO.
What can you do if you are on a High Population realm?
1) Try to run your dungeons early in the week.
Each realm will always accept the first 500 runs that are completed for a dungeon after reset, and the minimum rank will slowly increase as the week goes on. The earlier in the week you run your keys, then the higher chance these runs will make it into the Top 500 and be able to be picked up by RaiderIO.
2) Try to include at least a single person from a lower population realm
Even if your own realm is at 100% capacity in every dungeon, you could include one or more player from a lower population realm when forming your group which would then allow your run to potentially rank on that player's realm. As long as your run ranks on any of your group member's realms then RaiderIO will be able to detect it and award the appropriate points.
How can I see the status of my realm?
We've also recently added a special alert that will show on your character page in the Mythic Keystone section if any of your realm's leaderboards are at capacity.