Itunes & Wifi Syncing

Sometimes when I turn on my Itunes, my Iphone shows up on the corner and I can wifi sync it. But a lot of times, it doesn't show up and I have to plug it in to sync, even when the battery is way more than half way full. I want to know if there is a way to make my Iphone show up with plugging it in. I tried exiting Itunes and restarting it, but that did not work.