That may have been it. The movie just wasn't as funny as was the play.
The other problem with the movie is that the laughter is forced in many scenes. It's not one of Capra's best.
Yeah, that's what it felt like to me too.
It's been awhile since I saw it, but I remember Cary Grant lines that I felt fell flat, and the crazy guy yelling "Bully" also fell flat.
It was mildly humorous, not uproariously humorous as I remember it.