Baby won't latch to left breast

Anyone else got this problem? He's 3 weeks old and when he was born we really struggled with latching and used nipple shields and expressed. I've now got him latched on right side perfectly without shields but can't get him to latch on left side. He goes on for about 10 seconds and keeps coming off. I've tried different positions etc. I'm mainly expressing now and giving in bottle but it's hard work and I'm needing to supplement some feeds with formula when I don't have enough expressed.