pumping help

Rose
I'm back to work and am having  trouble pumping enough to leave for my  son. I am gone about 8 his a day. He is 10 weeks old and 11 lbs. He takes a 4oz bottle every 2 to 2.5 hours which amounts to 3 bottles while I'm gone. I try to pump at least three times during the day but I'm only getting a total  of about 5 Oz when I'm  at work, no where near the 12 ounces he drinks. Help!