It could be a camera problem that requires a tech. Since you are under warranty I would suggest calling Dell tech support.
x14 R1 is under a year old and under warranty. Face unlock worked perfectly until one day it just stopped working. The red blinking lights that normally show up when I attempt to sign in doesn't anymore. Windows Hello detects my face and sets up fine but barely works no matter how many times I try and improve recognition. This has been an issue for almost 6 months now. I've done the following:
x14 R1, face unlock not working, #2