why of 2 identical micro-controllers, only one can connect to a given device? I am working with 2 identical (Arduino) Adafruit Feather M0 Bluefruit LEs. Only one of them can connect to a Universal Windows Platform app running on a  Windows 10 machine. The other connection fails with Unreachable status.

I am only trying to connect with one MCU at one time. Both MCUs are loaded with the same program which resets the Bluetooth Unit. Both MCUs can connect to an Android app. Neither board appear damaged.

Why could a UWP app be able to connect to one board but not another?
Other than a software issue, there is no reason it shouldn't work. Only thing may be identical Bluetooth names or ids. Clear the device from windows and try again. 

I have some cheap Bluetooth obd 2 readers that all use the same Bluetooth mac address and name. I have to delete one from my android phone before adding the new one to connect to it.