I'm testing on a Nexus 5, with Android Studio, on a Mac. I used to be able to just plug my phone into my laptop, but lately when I go to run my app, it doesn't detect my phone.
In order to fix this, I have to go into the phone's Developer Options -> Select USB configuration -> toggle between all the modes until one of them works. Before it would just be MTP, but for some reason it seems to be pretty random which one actually does the trick now.
USB debugging is on. I haven't revoked any debugging authorizations... I know this isn't a huge problem, because I can just mess around with it to make it work, but it's super annoying since it does this every time my laptop goes to sleep, close Android studio, or unplug my phone.