The thing is that you’re using your phone as a light source, and on a sunny day its brightness is insufficient to give a well visible reflection.
Initially, the idea of the app was to facilitate navigation in low visibility — when you can't see beyond 50-100 meters. And bright sunlight is hardly a synonym to low visibility :)
If you’d like to use the app in daytime, we suggest that you try HUDWAY Glass. It’s a phone mount with a fold-down plastic lens. The lens is tinted and has a mirror coating — it removes image doubling and gives a better reflection on a sunny day. It’s live and selling at hudwayglass.com, Amazon, eBay, Fancy, etc.