Faces are tough, that's why portrait artists get paid a fortune
Mine are always yuk as well. The trick is light. Take a good look at someone's face sometime. It's not a solid flesh color, it's a combination of light and shadow. To make a model's face look somewhat realistic I think the best way is to try and recreate some of that light and shadow. Try some very light drybrushing on the cheekbones and forehead and maybe some darkening around the eyes.