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 doesn'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 frantically bash themselves to get the humans beyond the separating glass?
To the best of my knowledge, there is no in-universe explanation for this. Like almost everything else about the movie, the scene in question does not occur in the book.
The production history of the movie World War Z was particularly torturous and difficult. The third act (basically, everything from the plane crash through the end of the film) was rewritten a number of times, and the version we saw on our screens was wildly different from the original script.
As James Sheridan has already pointed out, the final version of the third act was written by a different group of people than the first two acts. This might have made it harder to achieve a sense of continuity and consistency throughout the entire movie.
I'm afraid that any other explanation would necessarily involve a significant amount of speculation: Maybe the zombies in the opening scenes were whipped up into a frenzy by the large number of people around them, whereas the WHO zombies were isolated in an area devoid of any humans for several days, and were therefore relatively calm. We just don't know, and it is difficult to imagine that we will ever find out.