cut and paste from help article on how trailer brake controller works...
You DO have a trailer brake controller don't you?
If you do, then I think you must be moving to make the trailer brakes work...
If when you tow, you should be able to "feel" the braking power of the trailer slow you down. If this does not happen then the controller may be adjusted to low or not hooked up correctly.
Trailer Brake Controller Information | etrailer.com
A proportional brake controller senses how the tow vehicle is slowing or stopping and applies the trailer's brakes with the same intensity. So if you slam on the brakes in your tow vehicle, the controller will activate your trailer's brakes just as forcefully. With a proportional controller, you can adjust the initial braking power and aggressiveness based on your trailer's weight and your braking preferences.
A time-delayed (or solid-state) brake controller activates the trailer's brakes with a preset intensity (power output) and rate of application (sync), both of which are determined by you. With this type of controller, there is a delay between the time that you initially apply the brakes in your tow vehicle and the time that the controller reaches maximum power output to the trailer's brakes. However, this delay can be adjusted with the sync setting.