Hi there Question3s,
Thanks for contacting the Apple Support Communities today! We understand you're having issues with your iPhone not installing iOS updates automatically, and we'd be happy to help!
Just to confirm, in Settings > General > Software Update > Automatic Updates, is both Download and Install iOS Updates enabled here? If you do, has this issue occurred with all iOS updates, or only certain updates?
We ask as not all updates will be installed automatically. You can find this, and more details in the following link under, "Customize Automatic Updates": Update your iPhone, iPad, or iPod touch
If this occurs with all iOS updates, do you receive any error messages when you notice that the update didn't download or install?
We look forward to hearing back from you.
Have a great day!