The first thing the app has to do to be able to measure is it has to detect a horizontal plane. In this case it is a floor. Here are some things you can do:
- Make sure you are pointing the device at the floor
- Move the device slowly forward and backward still pointing at the floor. You can even walk a few steps if you like.
- Try turning on more lights
- The floor might be too plain. For example, an all white carpeted floor with no texture. The detection needs to see "variances" in the floor to calibrate. Try moving to a different room or introducing some "variances" onto the floor (e.g. throw rug) that still maintains a flat surface.