If you are having problems with asset files not being included in your compiled apk file, it may be because your assets folder is placed in the wrong directory.

The assets folder should usually be inside the following folder:

That is the default, but make sure to check the asset folder configuration setting in your projects .iml file, to be sure which folder it really is:

<option name=”ASSETS_FOLDER_RELATIVE_PATH” value=”/src/main/assets” />

The path to the assets folder should be:

place all your assets inside, and the files should now get included in the build!

If not, try cleaning t he project, by going to Build -> Clean project
or running  ./gradlew clean