My Leftie is always a bit slower to let down and doesn't produce as much, doesnt seem to matter, by about 6 months DD2 was having about 80% ffrom right and occassionally having leftie. She did well and I fed until 11.5 months and supply was not why I stopped. DD3 is now doing a similar thing. My only issue was I did feel a bit lopsided at times as right was bigger than left, especially when I was full. Interestingly enough my mum was exactly the same but it didnt alter her ability to bf myslef and my brother.

I think in my cases the slower let down causes older bubs to prefer the other side, so there is less demand, so that side produces less, bit of a circle thing happening