4' flush-mount LED light won't dim
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.