I forgot to mention that Alex actually rings the doorbell several times when no one responds to his hammering on the door. But all of that is only after he finds the door locked. Though you're right, in the state he's in, it's surprising that he didn't just break the door down!!
And I also forgot to mention that Grayson's script has a couple notes. She writes that Carlotta should be slow and quiet and try to close the door in Alex' face (Alex wasn't about to let
that happen!
). And it also seems as if in her script she replaced the scripted "Mr. Jenkins! You must stop that!" (referring to Alex' shouting for Quentin) with "Gerard, Gerard, stop him!", but as we know and has already been gotten into in reply #473, what Carlotta actually says in the film is "No. Mr., Mr. Jenkins, please", which doesn't appear in her script or DC's.