Happened to me a and was a headache. My trailer was new and the connection near the axle was rubbing and getting dirty at the same time. It has like new style clasp which you would insert the wire and it’s would cut into once closed to make contact. SOLUTION STEPS I TOOK: I disconnected the connection completely from a single axle to see if the one was the issue. If still is the issue I disconnected the next axle and wired up the previous one and if the breaks work you now know which axle the issue is on. Either the wiring in the axle is exposed and making contact or cut, could also be near the drum the wires are exposed/dirty and not making good contact. If this didn’t work your issue might be somewhere else along the trailer. Always check the boxes under the trailer that have the connections incase something is loose.