iPhone Mirroring has always been flaky for me. Sometimes it works great. It does seem to "forget" whatever it needs for the connection from time to time (maybe if I don't use it in a while?) and then requires me to fetch my phone and enter my passcode so that it can pair again. ...Usually when this happens, my phone is out of reach and I have to decide if I want to get up and fetch it. I mean, my phone being out of reach is the main reason that I'd invoke the app at all...
And sometimes, I get stuck in a loop with the iPhone Mirroring app on the Mac telling me to unlock my phone and follow the prompts, but then when I unlock my phone it just immediately tells me that my phone has to be locked for iPhone Mirroring to work. :-\