One quick process of elimination that might be worth it, try connecting the working left signal to the right signal's harness to see if it still works. If it does, then at least you know its just a bad signal and you dont have to go hunting for missing voltage.
Just literally had this happen to me when I did my LED idiot light conversion. I ended up botching my right signal up somehow. But i didnt like them anyway and replaced with Lucas style.
