Yes, I had this on one side. Although I had 12V, the earth connection via the wingstay was poor. If you place a wire between bare metal on the wingstay and the main chassis (perhaps the headlamp shell), and the indicator now works, then you have the same problem.

I solved it by running a copper earth wire alongside the 12V cable to the flasher.

Simon

Same as you, flashing weakly, but strong flashing once Earth connection made.

good news