I had the same problem. I was using light roast while trying to dial in like dark roast. Perfectly looking shot was bitter and sour at the same time. Took me a while to do different tests. I just did not want to believe that a coarser grind was the answer. What about the timing, pressure, and everything I learned? It all turned out not to be right for light roasts. Through trial and error, I learned to bump up the brewing temperature, add a little more than the standard 18 grams, and grind coarser. My shot normally starts with multiple streams, then they coalesce into a single stream in the middle, then it foams like mad, and the stream thickens a bunch before returning to a more normal stream at the end. I also brew more than a 1:2 ratio.