I would check if the light conditions are ok, no bright back light, proper light from the front,
that the objects (face) is in the middle of the image and that you have the latest firmware version.
Besides that, un and repluging it might help as well to try it at a different location to see if the
light / background / ... was the cause for the trouble.
If you have an other cam it might be interesting to try that one to see if they behave different.