I came home yesterday and "Sync Now" was greyed out on both iPad and iPhone. No matter what I did I couldn't get them to show up in iTunes via wifi. As I thought about my configuration, I realized that I have my photos on an external HD. The HD is one of those that goes to sleep and there is sometimes a lag when it wakes up. I moved the pictures folder to the local machine and directed iTunes to this new location. This change seems to have resolved the wifi connection issue. Hoping this helps someone else.