BG has given some excellent advice!

My DD had a left side preference from very early on and I would only offer the right side first up. I discovered over the course of time that I had a much slower let down on my right side so DD had to really work to get her milk. By 14 months I was single side feeding from the left as she was really only using it for comfort

Perhaps your flow is slower on the right side too? Try using a warm facewasher to soften the breast and nipple a few minutes before she goes on?

Nae x x