The problem started surfacing a few months ago, even when I had my iPhone 5. (I recently upgraded to a Six Plus. It started happening with late version of OS 7; I have always kept up with OS updates! When I restored my 5 to my new Six Plus, it happened a few times, then after a reset on the Six, it has worked OK for three days, so I am keeping my fingers crosses and will post follow ups. Rebooting the iPhone did and does seem to allow WiFi synch immediately after the process. All my other hardware and software, including iCloud, WiFi, my iMac Yosemite etc. are working fine and are up to date. All other functions except Handoff work, but my iMac is late 2011 vintage and that is expected. My iTunes is also up to date, but this started with prior releases.
Maybe the issue is gone, but I am not sure yet...will update!