Help! Back to work and breastfeeding!

Has anyone had issues with keeping their milk supply up while working? I try to pump just as much as I would when my LO would feed but I am only coming out with at most 3oz a sitting. Before I went back to work I would pump atleast 5oz. Worried I will run out of frozen supply with out being able to replace it.. Also my job is very demanding I am constantly walking rarely in my office and can only pump 2 to 3 times a day. Please if anyone has advice. ( I have already tried milkmakers cookies to up my supply)