There is loads of information on this topic above and over here
If as I suspect it is an android device, which device you didn’t state?
I would check the app version again, as the DJI Fly app WILL auto-update unless you stop it
I feel for me the best and stable is 1.4.2
Also, force close all other apps, use aircraft mode and clear the memory
If that doesn’t work, try another phone and or refresh the Firmware
Let us know how you get on