You probably set up the rule to only add if there are more than one items in the cart. I'd have to see the cart file to know for sure, but that sounds like what you are describing.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.