My iPhone not locking at set time

Hey there, and welcome to Apple Support Communities raziya81,

We can certainly understand your concern and can help provide some insight.

We recommend checking out these step for more information on how to set a specific time frame for when your device locks and see if it matches up with your procedures.

" Change when iPhone automatically locks

You can set how long it takes iPhone to automatically lock.

Go to Settings  > Display & Brightness > Auto-Lock, then choose a length of time. "

For additional information, check out this link: Keep the iPhone display on longer - Apple Support

These steps will help ensure your device is working with ease and can fix common issues like the ones you’re experiencing.

Kindest regards!