....why is it so difficult getting a trailer set up properly?

Long story short, got new brake box controller. Had disconnection upfront in truck with one wire, got that all sorted out. Truck correctly wired and ready to go on its' end. Went to go hook up to our trailer had right side only on trailer lights working (brake and turn signal) nothing from left. All lights on truck working fine. But now we have nothing. Tried hooking our friends truck up to it, and nothing. While at the trailer place we hooked our truck up to an RV and the connection was fine. So it's def on the trailer end. With both our truck and the friends truck the brake box controller wasn't even lighting up. Again when he had ours to the RV the green light came on right away. And when our friend plugged into his 2 horse he had power right away.

Had the trailer inspected less than month ago and everything worked FINE. How does it go to crap just like that?! Haven't used it since the inspection.

IDEAS?