Windows Hello is a fast, secure, way to sign in to your Microsoft Windows device, and facial recognition makes it even easier. An important part of making it secure is that facial recognition needs to be confident that the face it's seeing in the camera is a live person and not a photograph or other image.
In order to do that facial recognition needs to have a clear view of your face. In some unusual circumstances the conditions may be such that it still can't be sure. If facial recognition is struggling to recognize you try the following tips.
Check your lighting. If there is harsh or particularly bright light coming from one side or the other, it may have a harder time recognizing you.
Some unusual kinds of make-up are visible to infrared and that can make it more difficult for facial recognition to recognize you.
Try removing any hats or other accessories from around your face. Glasses normally aren't a problem, especially if you were wearing them when you first set up facial recognition, but some types of hats or other accessories that frame your face could be.
If you've already checked these things and you still aren't being reliably recognized, then you might have a hardware problem with your computer's camera. Contact the manufacturer for advice, and possible repair.
Note: If your computer is managed by your work or school your IT administrator may have turned on enhanced anti-spoofing for Windows Hello. This sets an even higher standard for recognition and may especially cause problems in darkened rooms.