certain messages disappear after closing imessage

Hi kale226,

We'll be glad to assist you with this. Generally, messages should only be automatically deleted after 30 days at the earliest. With that said, what you've described shouldn't happen. If you're not on the latest iOS version, we'd recommend updating as outlined in Update your iPhone or iPad - Apple Support

"Update your iPhone or iPad wirelessly

1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. 
6. 
   
7. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.

"

If you've done that and still experience this, go ahead and reach out to Apple to look into this with you further: Apple Support

We hope that helps.

Cheers.