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.
If you are not getting any pull, and the lights are dimming, then you have a wiring issue.
In your first post, you mentioned having the controller connected to fixed voltage:
1 - Which wire numbering them from left to right 1, 2, 3, 4
2 - Which uncoupling track (O or 027) the model number would be helpful if you have it.
If you have an ohm meter check resistance of the magnets coil. They do burn out on rare occasion.
I had a similar problem with my original 1950's remote control track.
The center coil was fried!
I had to buy a new R.C. track.
The old one is used as a straight section.
Your wiring sounds correct.
Here is a diagram of what it should look like.
Another thing to check are all your other wiring connections.
Which transformer are you using?
Which terminal goes to the outside rails?
Which terminal goes to the center rail,
and which terminal goes to the UCS controller?
TO be clear: You've tried multiple controllers attached to the same UCS track?
By the way, I do not mean to discount the possibility of a bad coil, as others have suggested.
these posts are good to read. I had problems with the coupler magnet being to weak and that’s cause it was set up to run on existing track power so I cut off both wires to the magnet. one wire direct to 14volts accessory and the other going thru the push button controller then to the other side of accessory no problems with these if they are at least 12volts or higher as reported by lionel