Not emptying breasts

yinyang124

My boys recently have not been emptying my breasts and I end up having to pump after every nursing session. I have an overactive letdown, so they feed very quickly. They won't stay on for a second letdown though. I get about 1.5 oz each side after nursing. They were 6 weeks early, so I am giving them 3oz of formula/day to help with weight gain per pedi's request.

How can I get them to stay on? I try soothing them while they suck and wait for second letdown, but 9/10 times they don't make it. I'm frustrated and considering EPing since I end up pumping anyways. 

ETA: They are still hungry after first letdown, and I have to give them an oz of either bm or formula in bottle.