Siri will not make an appointment for me. Huh?

Hi,

Using iPhone 4S, with 6.1.3 OS. I ask Siri to make an apointment for me on my Calendar. We go through all the necessary steps until Siri says: "Are you ready for me to schedule it?" I say yes; Siri says: "I'm sorry, I'm afraid I can't do that." HUH?!!? Not a fluke - I've tried it five times now with no luck. I'm an iPhone newbie, so...be gentle.

Thanks for your help.