Express feeding

So my little one doesn't latch on, it's been two long weeks of trying, so I'm having to express feed but I really need advice on how to work around a solid routine of it where she gets only breast milk, the routine at the minute is that she gets one 4oz bottle of formula then on her next feed 3 hours later 4oz bottle of breast milk. Reason for this is because every 3 hours when I express (from both breasts) I only make between 1/2 oz. So I've had to get myself ahead with it so I can keep up with the 4oz's. Why is my breast not filling up, there's times I go 5/6 hours or more when out places of not expressing, and they don't get full. I express once back home and still only 1/2 oz. Now they seem to be slowly making less and less. My pump doesn't seem to have the greatest suction it takes in almost all of the nipple and areola (however it's spelt lol) could this be why? Should I get a different pump. Is my breast milk going to stop? How do I keep it up? What way could I go about changing this routine so that she gets only my milk? And no formula I really need advice