It happened to me that I had an instance of the packager running with an old project (I ran react-native start as usual). I was using Ubuntu 14.04. So what I did was to stop that instance and go to my project folder and in two different console tabs I ran these two commands separately:
npm start #here I used to run react-native start
react-native run-android
npm start is defined in my package.json as:
"start": "node_modules/react-native/packager/packager.sh"
I don't know if there is a sort of confusing stuff for react-native but that did the trick.