Sketching: Faces

Sometimes I find it really difficult to draw faces. 

So I picked up a free 'Stylist' Magazine in Town, and was sent a BooHoo catalogue through the post and I thought I should practice.

If you follow me on Instagram, you have probably seen some of these pieces.

I really like the outcome, and I think these faces are drawn well, I just need more practice on this subject.

What do you have trouble drawing?