There is a one-week (7 day) period in which you have to log in to your character for it to be included in the add-on. If you have not logged in and scanned your character in that window of time, then it will be considered inactive and removed from the add-on. To reactivate the character just log in/out of the character and then wait for the next Addon update. This will not be instant and depending on what version of Raider.IO you have will depend on how long it will take to update;
RaiderIO Revered Client User: Four times per day (00:00 UTC, 06:00 UTC, 12:00 UTC, 18:00 UTC) - Support us on Patreon
RaiderIO Exalted Client User: Eight times per day (00:00 UTC, 03:00 UTC, 06:00 UTC, 09:00 UTC, 12:00 UTC, 15:00 UTC, 18:00 UTC, 21:00 UTC) - Support us on Patreon
RaiderIO Client User: Twice per day (06:00 UTC, 18:00 UTC) - Download the RaiderIO Client
Curse/Twitch: Once per day (06:00 UTC) - Download from CurseForge or use the Twitch Client