Two reasons, generally:
1) Lack of energy. Comedy is often a dramatization of two or more colliding points of view. The verb collide is key there. It’s not two points of view that meet for a relaxed latte at Starbucks. It’s not two points of view sensitively recognizing each other’s essential humanity. It’s a collision. Sparks fly through application of force. Now that collision isn’t necessarily violent, it doesn’t mean that the energy I’m talking about means loud voices and sweat pouring down the brow and broken furniture all over the place. It just means there is a sharpness to it, an edge, there is electricity coursing through it – I often might tell actors that the scene must be “plugged in.” When it falls flat you feel as if the thing was simply never turned on. All the questions of “Evaluation” from Milton’s checklist are particularly relevant to comedy. That’s why the ol’ saying goes Dying is easy, comedy is hard. But if you look at those who do it well, you’ll almost always see actors who understand the energy underneath that fuels the collision in points of view.
2) Insufficient understanding of how the writing generates humor, and/or what makes this specific writer tick. This is a matter of experience and work ethic far more than it is of native intelligence or official education. An actor who does twenty comedy scenes in a few months’ time will be better at it than someone wading into those waters for the first time. Ditto for dealing with different writers. Sorkin is different from Durang who’s different from Simon who’s different from Shanley who’s different from Ephron and on it goes. The understanding of a specific writer’s voice and rhythms, when not instinctually present, can be developed through old fashioned hard work.
So, are there some actors who understand these elements better than others, who perhaps are more skilled at presenting these elements in the proper dosages to get the almighty laugh? Absolutely. Did they develop that acumen through a lot of hard work, reading, participating in a ton of comedic theatre or the like? Quite probably. Did they emerge from the womb gifted in comedy? Unlikely. Did they grow up in a family surrounded by others who understood these rhythms, and hence picked it up through years of that kind of experience? Yup. Lots of ways to skin a cat, to get this comedic sense in your acting. But I think too many actors rely on the generalities of “I get it” or “I don’t get it” to assess their abilities with comedic material as a whole, or this week’s three-page comedic audition. The fact is it can be a developed ability, and the material itself understood through application of work and developed understanding.