Sounds like you might not have a good ground on that RL. If the ground is poor the bulb can illuminate a little, but not always full illumination. If the bulb doesn't fully illuminate it wont trip the flasher and you end up with the situation you're in. For me, differences in brightness are easier to see in the dark. I would see if you notice a difference in brightness between RR and RL. If there is a difference, the ground is likely the issue.