Q: iphone 6S Plus

Hello there igoh777,

It sounds like you are losing the ability to delete individual texts after a little while but quitting the Messages app will bring it back. If this happens on all of your message threads then I would next suggest that you backup your device to iTunes, then restore it as new and verify that it works consistently. 
 
How to erase your iOS device and then set it up as a new device or restore it from backups
 

If it does, then restore your backup to either verify it still works and the software just needed reinstalled, or isolate the issue to the backup file itself. 

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