Why don't the WHO zombies franticly bash their heads through glass like earlier zombies?

Early in the film, we see zombies headbutt car windows in order to reach their prey, but the ones inside the WHO facility do not seem to use that tactic. It is possible that some of the facilities glass is shatterproof, but the broken glass strewn about the place proves some of it is not. Furthermore, the female scientist zombie dosen't show the signs of head trauma one would expect if she had encountered glass she couldn't break through with her forehead. I may have missed the reason in the film, or maybe the book holds the answer to Why didn't the latter zombies franticly bash themselves to get the humans beyond the separating glass?