Sometimes this happens to me when Android Studio
is not running new code. I usually cannot pin it down on one specific thing (ADB, Android Studio, Build process, etc) so I go for the scorched earth approach.
Uninstall the app via ADB
:
adb uninstall <package_name>
Restarting Android Studio may help, but also running a Build > Clean Project
If you have changed any Gradle settings or files you may have to sync those up, so try Syncing Project with your Gradle files
.
If this all really doesn't work, a restart of your emulator/physical testing device and computer may be in order.