Help pumping !

India

Soo my daughter is 7 weeks and I had to give her formula because her weight had dropped. I guess I wasn't producing enough milk at first. Around 4 weeks she was back at a good weight but when I try pumping nothing comes out & I know I have milk (I can express easily) i have 3 different pumps and no results. Any ideas why or how to fix it? I'm kind of beating myself up over not being able to give her breast milk.