I have the same problem using True Key facial recognition to log in on Chrome on macOS High Sierra. Resetting (by going through face scanning again and make sure I use my normal intense expression) didn't help. I use a 2014 Macbook Pro 2.6 GHz Intel Core i5 with 8GB 1600 Mhz DDR3 memory. This is my second year using True Key Premium Edition and my experience with log in using facial recognition has been inconsistent.
Login via Facial Recognition on my iPhone 6 has been consistently good on my iPhone 6, probably because the screen's significantly brightens to light up my face from the front. Except on a few rare occurance, the True Key login in page (after I click on the 'I am ready' button) DOES NOT light up. Supplemental lighting from a desk lamp or a nearby window does not help, probably because the lighting is from a slightly different direction or level. It creates visible shadows. Occasionally, I see the login in screen brighten up for a second but dims back down way before the facial recognition completes. Disabling advanced facial recognition (where you have to turn your face to the left, then to the right) helps a little. I also have not found a preference setting to make the screen brighten up for this process. This happened to me 2 days ago. I solved it by uninstalling the True Key Chrome Extension, reboot my mac, reinstall the extension. I picked up this trick after a couple of level-2 tech support help on a similar issue.