The springs for my portafilter need to be replaced. The guide (Repairing Starbucks Barista Portafilter) says Step 3 If spring...
I had a similar issue. I found if I moved the portafilter to the right and held it there, it would flow well, if not the pressure would mount and would not flow. Check your springs in your portafiler, and take it apart to clean it a bit.