Board Thread:General Discussion/@comment-168424-20180709154300/@comment-168424-20180710182942

TheKorraFanatic wrote:

IIRC, you can't edit Discussions posts as a non-administrator or Discussions moderator anymore twenty-four hours after its creation.

Ahh, that must be it. It is mentioned in Help:Discussions, but it's easy to overlook and is a strange restriction. It's an especially bad restriction for limited time contests or events announcements...