thusband I will make the assumption the pressure gauge is showing at least 9 bar brewing pressure?
It’s unlikely that for some reason the valve isn’t opening enough, because there is nothing mechanically that can “slip” and make that happen, unless you’re paddle isn’t properly on the splines.
I am going to guess that you don’t remove the valve mechanism regularly to lubricate it….I’m also going to assume that the machine is at least 6 months old or older? Based on all those assumptions, I’m going to guess that the jet might be blocked by a bit of shredded O ring. It could of course simply be blocked by something far less terrifying.
Remove the flow control valve from the top of the group, screw out the central core, photograph everything, including down into the group jet. If 1 or more O rings are shredded, they need replacement. Lubricate the valve and replace it. Use a spanner with jaw protectors, or make some up. You may need a small “socket on a stick” to get the group jet out.
you may have to remove the group jet itself and clear any blockage…..
When replacing the valve be super careful to only screw it in with your fingers and not the paddle initially, so check the pin is properly lined up with the hole in the group jet! Watch the videos very carefully. Lubrication maintenance of this valve should be done every 3-6 months….and if fitting one, lube it first as i did (Dow Corning Molykote III, I have shares in this 😉). All the information you need is in the 2 links below.