Why is force restart not working on my iPhone 16 Pro?

Try it more rapidly.

Press and quickly release the volume up button

Then immediate press and quickly release the volume down button

Then immediately press and KEEP HOLDING the side power button for as long as it takes to see the Apple Logo

It's often more successful if you do it this way.

Press and release volume up, then volume down, then press and hold the side button until the Apple logo appears. This can take 20 to 30 seconds in some cases.

If something isn't working, you need to explain what you tried and what happened. We are not psychic. "all necessary steps" is not informative.

It is all in the timing and I have never seen where it has not worked unless there is a hardware problem with one of the buttons that is not responsive. The sequence of your key presses is correct, you will just need to adjust your timing and the press and release of the volume buttons must be performed quickly.

If that isn't working, and the device is completely unresponsive, make an appointment at the Genius Bar of your local Apple Store.