Just bought new IPhone 11 pro

Hi I just bought my new iPhone 11 Pro from you a month ago. I love it, but 2 weeks after having it I noticed that after I shut it down and tried to turn it back on it took multiple tries. Thought maybe it was adjust a fluke, until yesterday. I tapped the power button to make my screen go black....power saving mode...on stand by...not sure what it is called. It wouldn’t go black. So I shut it down and I took me multiple tries to turn the phone back on again. I take good care of my phones. It has not been dropped. Why is it doing this?