I've had a Gaggia Classic for a few years and over the last few months I've been having issues with the flow from the group head.
I think it might be an issue with the solenoid valve. There was some guidance I saw online about flicking rapidly from the group head to the steam wand, as this forces water through and clears any blockages. And yes this does seem to work - if I rapidly flick the switches the water suddenly flows a lot faster from the group head. But it's not a permanent fix.
I've descaled it several times. Is there something else I can do, or does the valve need replacing?
I wasn’t using the correct water, and after about two years I needed to replace my valve. After inspection my boiler was also beyond descaling and the steam valve was drippy. I replaced those parts because I could but only the solenoid valve was required. FWIW there are videos on how to clean the valve (after removing it) but they didn’t work for me.
I used this, but I’ve installed the shades pid, shades pressure gauge, and a dimmer mod so I was fairly familiar with the innards.
Consumer carbon filters don’t really remove enough hardness to be effective. For convenience I use distilled gallons and third wave espresso packets. There are cheaper methods though.