iMessage is not functioning properly I just updated to iOS9 and all hell broke loose on iMessage-every now and then, messages that I receive would pop up on the screen for a second and then disappear! I have tried restarting iMessage multiple times but the issue seems to persist. And this has been occurring only since the update. What must I do to rectify this situation?
As one potential route to take, IPSWs (firmware files) for iOS 8.4.1 are still being signed as of the time of this posting (according to ipsw.me). If you act quickly, you could try downgrading and see if the problem persists.
From an article by Realty Today:


Back up your device data and settings so you don't lose them when you downgrade. There are two ways to back up. First is via transferring them to your computer. Plug in your device to a computer, open iTunes, and click Back Up Now on the device page. The second option is via iCloud. Go to Settings, iCloud, and tap the toggle button to turn on Backup. Ensure that you have enough space for all your data. Apple provides 5GB of free storage only.

Download the iOS 8.4.1 IPWS file that corresponds to your device model.

Plug in your iPhone to your computer, open iTunes, and go to the device page. Press Option/Alt on your keyboard (for Mac OS X) or Shift key (for Windows), then at the same time click Restore iPhone.

When the explorer box popped up, select the iOS 8.4.1 IPWS file you've downloaded then click Open to start the installation process.

When iOS 8.4.1 has been installed, iTunes will prompt you to restore backup. Click Yes to have your data and settings back.