Like Ninjamyst said above, if you didnt change anything except the membrane, then you should try pushing it in more...lube the orings a smidge with silicone oil. Could also be a checkvalve or a few other things mentioned, but start with the membrane. I also had this issue after 2 or 3 years in...got a new membrane and thought it was in. The cap even went on "correctly" Took me 2-3 membranes before I figured it out, but based on the rest of my years of exp, I can say that you did not get 3 bad membranes in a row.
If you're positive that it isnt the way it's inserted, then pull every line loose at one end, turn the water on slightly and when water comes out of a tube, connect the loose end and move to the next....that will show you where the blockage is
If you're positive that it isnt the way it's inserted, then pull every line loose at one end, turn the water on slightly and when water comes out of a tube, connect the loose end and move to the next....that will show you where the blockage is