Be sure the IGNITION KEY is on when doing the lever test. Also some trucks like my 2010 FORD F150 has to be going at least 10MPH when just pressing down on the brake pedal to get any BRAKE CONTROL action going to the trailer brakes... The BRAKE CONTROLLER SLIDE LEVER however should work all the time but ignition key may have have to be on first.
I always check my trailer brakes when I am first pulling out for the trip. Will use the BRAKE CONTROL LEVER coming up on the first STOP SIGN in my neighborhood and let the trailer brakes stop me.
Roy Ken