Yes, it can be a simple as the shower screen getting just barely gunked up enough to alter the drip pattern of your group head. IMO, this is the single most archaic and inconsistent aspect of espresso machine design and one that I can't believe no one has tried harder to address.
Even the level of torque applied to the shower screen nut has an impact.
There could be other variables in your case, but this is the most common reason that I experience the symptoms you describe.
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