Letdown help!

I’ve been EP for about 3 months (4 months post partum) and I am having a HARD time getting letdowns. They happen fine the first 15 minutes, getting 2. After that I can pump all I want and not get any more. But my breasts are still full. Massaging hasn’t helped. Last night I legit pumped 35 min with NO letdowns and full breasts. When I hand express milk comes out. Its happened the past 2 pumps in a row. I’ve varied the speeds of my let down phase on the pump and still nothing. I changed my membranes this weekend. Help!