Iphone 10R tap screen to wake up is not working all the time

When you raise your iPhone to look at it, it automatically wakes the lock screen. ... On iPhone 8 or earlier, press the Home button. If you don't do anything, your iPhone will go back to sleep. To turn Raise to Wake on or off, go to Settings > Display & Brightness.

Use Raise to Wake on your iPhone - Apple Support

Turn on Back Tap

1. Check that you have the latest version of iOS on your iPhone 8 or later.
2. Go to Settings > Accessibility > Touch, and tap Back Tap.
3. Tap Double Tap or Triple Tap and choose an action.
4. Double or triple tap on the back of your iPhone to trigger the action you set.

Use Back Tap on your iPhone - Apple Support