I know its a stupid question, but have you changed a bulb on the passenger side, they should be 21 watt bulbs,
If you have fitted a 5 watt side lamp bulb by mistake it would give the symptoms you have.
Since the flasher unit serves both left and right flashers, and one side is working i would asume it is not the flasher unit.
I have also come accross where someone fitted a double fillement bulb 180 degrees out, gave then a nice bright side light
but a dim flasher.