Some phones (e.g. Samsung) STOP collection of Bluetooth Signals when the screen is off.
As can be seen below for a Samsung phone, Bluetooth is not collected when its screen was off.
Citizen Synergy Android App has a special NO SCREEN OFF feature.
If you keep the app displayed on your screen, then your phone's automatic screen off will be disabled - allowing Bluetooth signal collection to continue on those troublesome phones that switches Bluetooth scans off when screen is off.
Note this is OPTIONAL we should have enough data from continuous collection of GNSS, WiFi and Cell Tower signals - which still happens when the screen is off.