Rocket Giotto tar buildup
I've had my Giotto 2 years now and am very happy with it. I've scrupulously backwashed it monthly with espresso machine detergent as per the manual.
When the group head started leaking I decided it was time to replace the gasket.
I was astonished when I removed the filter and gasket to discover a layer of tar, for lack of a better description, several millimetres thick blocking most of the holes at the back of the filter. Despite soaking it in boiling detergent, it required the use of a flat bladed screwdriver to remove it. I then spent the best part of an hour with wooden skewers and toothpicks to unblock the holes.
Long story short: If you have one of these machines. or a machine with a similar head, periodically remove the seal and clean the back of the filter, as monthly backwashing with detergent does not remove it.