Check with a meter to make sure you have continuity through the brake circuit on the trailer side. You can also do a basic test of the trailer's brakes by pulling the breakaway cable (with a charged battery in the trailer of course) and checking with a compass at each wheel to make sure the magnets are energized. Be sure to put the breakaway insert back into the switch when you're done with your check.
If you have an open circuit at the brakes I don't think the controller will "know" there's a trailer plugged in.
Il Carriaggio 1975 Argosy 24 | Il Progetto 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. Sir Winston Churchill