2 arduino connected to USB

i have 2 arduino cards connected usb, com5 , com6.  2 days ago both can seen arduino ide , listed in device manager.  can communicate both of them.   today, 1 can seen arduino ide device manager still can see com5 , com6 used arduino cards.  both cards' lights on. 

but 1 card can communicate com port.

what problem?

thanks

first try futher isolate symptom.

swap 2 arduino boards. problem move board or stay physical usb port?

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