Not latching on one breast

I've had a really tough time breastfeeding. My son won't latch all the time so I pump and feed through a bottle. When I do get him to latch it is amazing... but it is only on my right breast. No matter how I try or how many positions I put my son in he will not latch into my left breast. He screams and cries and it breaks my heart. But when I switch him to the right side (most of the time) he latched no problem. Any ideas on what I can do to fix this?