Have you soaked/cleaned the actual group handle?
When I do the water backflush weekly, I also soak the group handle in a small plastic bucket filled with a little espresso cleaner and warm water. I just stand the handle up so that only the metal bits are under water, leave it for about 30 minutes and then rinse thoroughly. I also put any of the baskets Ive used during the week. Im always amazed at how shiny everything looks after such a short soaking.
Perhaps the group handle has got some build up (in spouts perhaps). Give it a soak or failing that, try to get some pipe cleaners (or similar size things) and see if you can scrub the spouts out.
Hope that helps.