Breast pumping help :(

I have a medal double electric. This is my 3rd baby & pumped for both the 2 before. I'm superrrrrr full but it's like nothing is coming out. When I do it with my fingers it comes out, but with the pump it comes out just a little then it's like it stops/is blocked/stuck. I tried a heating pad tonight & it didn't seem to help much. What else can I do? Why isn't this working this time?