Android Studio is feeling a little lazy and not really doing the whole building thing before you run or debug tryy these......
Check the Gradle Sync Thingy: So, like, you can click on "File" > "Sync Project with Gradle Files" to manually mess around with Gradle sync.
Go over to "Run" > "Edit Configurations" or something like that. Pick your app's configuration thing in left. Look for "Before launch" section,and click on "Make"or the '+' thing.
Cleaning and Rebuilding: Click on "Build" > "Clean Project" Then, do the "Build" > "Rebuild Project".
You could, like, update Android Studio if you want to, but then again, why bother? Older versions are kinda cool too, in a retro way.
Make sure you've messed with your code a bit before you hit "Run" or "Debug." If you haven't, Android Studio might decide to take a coffee break instead of building stuff.
I think there's some Gradle configuration thing hiding somewhere. Check it out if you're in the mood. It might have some auto-build stuff that you could turn on or off or something.
Custom Build Scripts? Maybe If you've got some fancy custom build scripts, well, they might be doing something funny. Or not. Who really knows?
Third-Party Plugins, Whatever:
If you've got a bunch of third-party plugins cluttering up the place, maybe disable some of them? Or not. It's your party.
Take a peek at your Gradle scripts, XML files, and other random config files. There could be errors or something. Or not. Who's got time for that?
If all else fails, restart Android Studio. Or don't. Maybe it'll fix things, or maybe it won't. Life is full of mysteries, right?