Thanks for posting your question here! Plugable UD-3900 has a blue power LED that lights up when it is being powered by the power adapter.
If the blue LED does not light up when the power adapter is connected tight to the Plugable UD-3900 and is plugged to a live power outlet, then it is an indication that either the unit or the power adapter is faulty.
If that is the case, please do shoot us an email at email@example.com with the Amazon Order ID through which the product was ordered and the serial no (given at the bottom) of the dock and we can get the issue resolved.
Hope the above information helps! If you need any additional information, just shoot us an email at firstname.lastname@example.org. We are here to help!