Default [Q] Android App not installed

Okay so I am trying to reverse engineer an android app, whatever you want to call it. Anyways I get the apk, and I rename it as a .zip. I use the dextojar to change the classes.dex to a jar so I can use a decompiler to look at the code. I found what I wanted to change so I used a smali converter and changed the classes.dex to the smali files.. changed the smali to my liking then converted back to the classes.dex, put it back into the zip and then back to apk and the phone itself.

I have done this before, it has worked before... now no matter what app I do this to, it doesn't work? I tried on a nexus 5 and a 2013 nexus 7

Is it because of the android update? Any ideas would be helpful...

Thanks!