If the diode you speak of is in the dashboard circuit, I don't think you need that with dual dash lights. I think it is used with one dash indicator serving both turn signal sides. The one side flashing only could be dirty connectors, or not enough current being drawn through the flasher relay. Are all the connections clean? If the bike is running and you raise the rpm's, does that help?
Last time I addressed this issue it was for one dash indicator, and took two diodes opposed to each other to make the system work properly. If the turn signals are led's they may require an electronic flasher. The new flasher relay doesn't need the load of an incandescent bulb to make it function.
