A human head (or a complete human) is a really difficult thing to model. That's because we're so used to see them in real world every day. Even slightest misplacement or erroneous scaling of some parts will show immediately. I've learned this during the conventional drawing course i'm taking atm. We're doing a charcoal drawing of a plaster head model and it's really tough one to crack.

I don't say I'm an expert or anything but here's what i think is wrong with your model (without looking the reference images):
1. From the side view it seems pretty ok except i'd move the ears a bit towards the face
2. The eyes are a bit far away from each other
3. The nose seems a bit wide (and thus it seems too big in general)
4. The outer corners of the eyebrows could use some more definition. Maybe pulling them out from the head more...

Don't get the wrong impression. I think the head looks very good as it is now. I know I couldn't do anything like that at least