Try another power supply just to make sure the wall wart is not the problem, but I have seen this in a number of Uniden scanners. They will not power up, you hear a clicking noise not even though the speaker and the display may flicker. This is usually due to a faulty component in the scanner.
There is an internal component that shorts or will not "boot" and can cause this. Seen it more with the 996XT platform.
The crappy part is the cost of repair is close to the cost or higher than a used 15X scanner these days. I think by the time you ship the scanner to Uniden for repair you are close to $90 or so for repair?
Not sure if the basic architecture and layout of the 15X is similiar to the 996XT, but I recall when one of my 996XT had a similar problem these components were replaced.
IC21 & R162
The 15X may have different component numbers but IC21 may be a common part between the 2 platforms????