I used to work with this application: https://play.google.com/store/apps/details?id=com.fakegps.mock&hl=en And before this with this one: https://play.google.com/store/apps/
In Settings -> General -> Location -> Mode
I switched it from Battery Saving
to High accuracy
and then the mock location app started working.
Note, you also need to set your preferred Mock Location app in Developer Options, but even after doing that my app was showing my true location instead of the mocked location - now it's working like a champ!