If the blue power light on the front of the unit is flashing, this indicates the unit is not getting the power it needs to function properly. The first things to check in that case are as follows:

• That the wall power outlet (or power strip) is working properly. Switching to another outlet or power strip can sometimes help
• That the ‘prong’ that attaches to the power adapter and provides the 'blades' that insert into the power outlet is securely attached
• The power adapter is securely connected to both the dock's power input and to the wall power outlet
• That the correct power adapter model for the dock is in use, as on rare occasions a different power adapter may be used accidentally. Over the life of the product, two different styles of power adapter have been included with the product. Photos of both styles of the correct power adapter for the UD-3900 are shown below 

Power supply style #1

Power supply style #2