Face unlock doesn't seem to set up.

[Seiranians]

I'm trying to unlock my phone using the face unlock on my HTC Vivid, but whenever it reaches the "setup" screen, it never detects my face in the little circle. The check mark never shows up. I even tried to change the lighting and change the room I was in and still nothing. Can anybody tell me what I'm doing wrong and help me? Thanks in advance .