It should absolutely reconnect automatically... I'm wondering if Bonjour is playing up on your iPhone or Mac. Have you tried rebooting both?
Also, if your Mac has a static IP address on your network, you could always configure it as direct connection which bypasses Apple's device discovery stuff.
One final thought... could your Mac be going to sleep? When the mac wakes up, it can take a second or two for Bonjour to kick in again.