There is a fight going on between your dimmer and your lights, and the dimmer lost.
One of the (too many) bits of electronics in your dimmer died.
Dimming is not as easy as it seems it should be, well, dimming without huge size, weight and heat.
In dimming led's, you are turning them on and off, fast, really fast, hundreds (or more) times a second fast.
For each of the on/off cycle, there are parts having to do work in the dimmer. Everytime electronics turn on, they make heat. Heat is the fastest way to kill electronics.
Your dimmer likely died in its really dim setting, and simply is not going to change.
Dimmer is gone - wants to be replaced
I agree with previous poster to experiment with different bulbs. I have a different circumstance (older dimmer switch with track lighting) but the symptoms were similar for me. I was hearing a maddening low hum unless on full blast. I found that LEDs labeled as dimmable or regular incandescents work fine. It was an off brand LED that finally worked. Also consider that newly built does not always = correctly built.
Best Answer
Test the working dimmer on the light that won't dim... And test the non working dimmer on the light that you know will dim... to verify its not a bad dimmer or fixture.
Also try swapping the wires on the dimmer.... some dimmers require the hot to connect to a certain pole and the switch leg to connect to a certain pole. Some dimmers also have an optional third wire for a three way switch configuration, be sure you aren't using that one.