Have read that there is a time-delay built in to the IBC for low-speed braking, to prevent neck-jerking/wheel lockup. The controller for my '09, and an '04 and '07 before, all work at low speeds, but not immediately.
Try using the manual lever and hold it on--you should get brake action if everything is working. If not, you should be getting a message on the dash center saying "fault in trailer wiring" or "trailer not connected"
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died