Trailer Brake Activators

[ARARAR]

I'm having problems with my trailer brakes and am wondering if anyone has had similar problems. I have an Activator II, made by Draw Tite. I have no problems setting it up and it seems to work fine at initial set-up; you know driving at 20mph applying the brakes, setting the aggressiveness of braking power. It's only after I'm on the road at cruising speed that I have the problem. When I apply the brakes, there's nothing there and it feels like all I have is my service brakes and I get an overload indication on my readout. But when I am finally coming to a stop at about 25mph the trailer brakes start working, I can really feel them. I try to set them up so the trailer brtakes just before the vehicle does. What's up with dat? I've checked the wiring and voltages and all's well. The unit even has a display that tells you if the wiring is right or wrong. I had the trailer brakes adjusted just before I bought the unit. And the unit bench tested OK.