Based on the symptoms you described, it's possible that the issue is caused by a clogged or partially clogged shower screen or group head dispersion plate. These parts are responsible for evenly distributing water through the coffee grounds and can become clogged with coffee oils and debris over time.

In addition to descaling, I would recommend removing the shower screen and group head dispersion plate and cleaning them thoroughly with a brush and a cleaning solution designed for coffee equipment. You can also try soaking them in a solution of hot water and citric acid to help dissolve any buildup.

It's also possible that the issue is caused by a worn or damaged pump, which could affect the pressure and flow of water through the machine. If cleaning and descaling do not solve the issue, it may be necessary to have a professional technician take a closer look at the machine.

Lastly, if you have made any changes to your coffee beans or grind size, it's worth noting that this could also affect the quality of your shots. Make sure you are using fresh, high-quality beans and experimenting with different grind settings to find the sweet spot for your machine.

To fix a clogged screen, you can remove it from the group head and soak it in a cleaning solution for a few hours or overnight to dissolve the buildup. You can use a specialized coffee equipment cleaning solution, or make your own by mixing hot water with a small amount of citric acid or vinegar.

After soaking, use a soft-bristled brush to gently scrub the screen, being careful not to damage the mesh. Rinse the screen thoroughly with hot water and reattach it to the group head.

It's important to clean your shower screen regularly to prevent buildup and ensure optimal performance of your espresso machine. Depending on usage, you may need to clean the screen every few weeks to a few months.