I have a feeling it's there's a difference in the amount of charge going into them...one has more power surging through than the other? Best thing to do is, swap the side that's working normally onto the side that's not and see if this problem persists... by dong this, you'll know if it's the bulb or the actually connection of one side over the other - I would bring it to a mechanic... one of my headlamps didn't shine as bright as the other and I just took my car to a mechanic and they fixed it up in like 10 minutes cost me $10.00 so yeah, I wouldn't worry about it as it can be fixed cheaply!