Try holding a metal screwdriver just above the center of the round red electromagnet in the center of the track while pressing the button (with the power to the track on). You should feel the screwdriver attracted to the electromagnet. (about 1/8 inch or less above).
It is not unusual for light to dim or trains to slow down when you press the uncouple button. Those uncoupler electromagnets draw quite a bit.
the uncoupler armature has to be right above the magnet in order to open the couple.
Let us know whether you can feel the pull, and we'll take it from there.
