Hey, I wouldn't bash these -- it's definitely a good start, yo. And it's even more awesome by the fact that your artist's eye is making you hyper aware of how you construct faces. I think your strongest differences here are in the first, the third, and the fifth. They've got great variations in the chins, eyes, and overall contour
Just push those shapes more, keep experimenting with the contours. Build your face-vocab up from rough studies of basic shapes -- circle, square, diamond, oval, heart. Once you're comfortable with that, start mixing and matching shapes. Heart+circle, Square+diamond, etc, etc. In the example below, you'll see how simple shapes can bring out different looks and character designs!
This is from Lucy Marie Illustration>>