First, use a test light or multi-meter to check for an open or improperly grounded circuit, or for voltage from the vehicle. Lack of vehicle voltage means there is a problem with the vehicle wiring or ground. Another method is to substitute a known good light.
When mounting LED lamps to trailers, don't rely on the “frame” or “chassis” as a grounding point for lights. Run a dedicated ground wire from the light(s) to the plug that connects the trailer to the tow vehicle. This ensures lights have the absolute best ground connection possible.
One wire can be used to interconnect the ground wires of all lights. That wire can then be routed to the front of the trailer.
Your tow vehicle should also have a ground wire running from the trailer plug to the frame of the tow vehicle. Don't rely on the hitch ball and trailer coupler as the only method of supplying a ground connection from the tow vehicle to the trailer.