There could be a number of reasons. Maybe the team is striving for higher quality, or there could be challenges in coordinating the various aspects of the production like voice acting and music.
Well, it might be that your story didn't meet certain formatting or content guidelines, and it's going through additional review. Or, the server could be slow due to heavy traffic.
There might be some technical glitches with the Messenger app itself. Try restarting the app or your device to see if that helps. Another possibility is that your device is running low on memory or has other background processes consuming resources, affecting Messenger's performance.