Is the turn signal voltage present at the plug on the tow vehicle. If you have no voltage there, the turn signal on the trailer can't work. It is also possible you have the bulb turned 180 degrees making it wrong in the socket. Yes -- the bulbs are keyed so they only go in one way but sockets are made so cheaply it is really quite easy to insert a bulb the wrong way. Hold the bulb with the brass end up so you can see both nubs. You will note one is located higher than the other. Make sure you get the bulb in correctly. If you have to force it, you are probably doing it wrong.
There are two seperate filaments, like two seperate bulbs inside a tail light bulb. One can work, and the other not. The reason could be burned out filament, bad wiring, or supply missing. (like silvertip said) The trick is it is really like troubleshooting two seperate bulb circuits.
you have 2 wires going to the bulb, tail light, and stop, turn light, you could be wire wrong on the vehicle, and need the lighting converter. it ties in the turn signal into the stop light wire, so the turn signal works. What Vehicle????
What made you change the bulb ??? blown filament or the same thing you are experiencing right now ????
If same thing and they were previously O.K. then check all the grounding first.
