“Hey Siri” doesn't always activate when screen off

1. “Hey Siri” is turned on my iphone11 pro,
2. Always listen for “Hey Siri” is also turned on
3. Also I am on the latest version of iOS

Despite all these features turned ON, "Hey Siri" doesn't always activate when screen is off

”Hey Siri” starts working when I tap on the screen once and the screen turn on,

then the whole problem disappears for some time and my phone starts responding to “Hey Siri” even when the screen is off.

Then this problem appears again in few hours or may be the next day and stops responding to “Hey Siri” when the screen is off.

Anyone else experiencing such a problem?