One breast not responding to pump - advice, please!

Kate06

Hello!  I just started pumping in the morning and evening a few days ago.  While my right breast is a champ and has no problems with let down or emptying, my left breast will have nothing to do with the pump.  I've tried massaging, letting the baby initiate let-down, etc., etc., but I get no more than a dribble, maybe if I'm lucky a couple good squirts, from the left side.

 Any thoughts on how I can troubleshoot?  Do I possibly need a different sized flange for my left?  

 I don't know if it's helpful to know, but the baby greatly prefers the left side, so I know there's milk in there!  Could it be the breast is just accustomed to baby(excuse me if this is crazy talk - new to this! ). 

I wouldn't be so concerned about this except I would like to pump upon returning to work, and if I've got one breast that won't empty for the pump that can't be good! 

Anyway, thanks in advance for the advice!