Welcome to the forum.
I hope this helps a little...
Is this a new or used trailer? For new trailers, there is a brake break-in process in the trailer's manual.
I believe, too, that some brake controllers reduce the trailer braking at very low speeds for smoother stops.
After that, it becomes a troubleshooting issue: Are the trailer brakes getting voltage, is the trailer wiring to the brakes OK, is there an incompatibility with the truck's brake controller and the trailer, etc., etc., etc.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.