There could be lots of reasons why the camera won't let you take a photo in iA mode. If you're too close and the camera can't focus on the object you want in Macro mode, if you don't have enough light, if the previous photo hasn't finished writing to your card, etc., etc. Without more details on what you're trying to photograph and your setup it's difficult to say what could be the reason.
When you shoot in iA mode you're at the mercy of some software programmer who made those decisions for you years ago. It's best to start weening yourself off iA mode as much as you can so you can start taking control over your photography. Start my just switching to MF, then slowly migrate to the other modes that give you more control - P, A, S, and M.