pumping issue

so I have been pumping milk for almost 2 weeks now. I normally get 4 oz within a 15 min time frame... but the problem is, after 15 mins of pumping the milk stops each time and I can't get anymore milk to come out. 4 oz is ok right now where my daughter is only on 2 oz of formula (pumping and dumping milk now due to medication, should be able to feed her to breast milk starting sat when the prevpac is out of my system completely), but I'm worried about the future for when I have to up her intake, afraid I won't be able to pump enough. any suggestions on how I can help my supply? what size shield does everyone use?? I think mine is the 24mm.. maybe I need a new size, it doesn't feel like mine is sucking hard enough, even when it does the let down it doesn't feel powerful enough and I got it as high as it will go.