I never own a Leica camera in my life. Only owned a Nikon FM2 which I got it from Cathay Photo somewhere in 1989. I shoot a lot of wedding photos, products, foods, sceneries, and commercials.
From my FM2, I managed to own an F3, F4, F5 and D100...
I guess that there's nothing wrong with the batteries or your flash light. My advise is to refer to your "flash instruction manual". Even though you've selected an "A" mode on your flash light and it still fires at full output, it might be your subject distance to the flash light is out of range.