DIAGNOSIS This usually points to the brew group—the part that holds the coffee puck—not building enough pressure. The most common culprits are a worn-out seal/gasket or old, clogged shower screens, not the grind.
FIX Start with a deep clean of the brew group. Remove it (usually by pressing a lever or button to eject it). Rinse it under warm water. Pay special attention to the two metal shower screens (one on the machine, one on the brew group). Use a small brush, like an old toothbrush, to scrub away any baked-on coffee oils. Let everything air dry completely before reinserting. Also, check the rubber seal (gasket) where the brew group locks in. If it's cracked, hard, or flattened, it needs replacement.
IF THAT DIDN'T WORK If cleaning doesn't help, the issue might be with the pump itself losing strength, but that's less common. Before assuming that, make sure you're using enough coffee. Adjust the "coffee quantity" knob to a higher setting so the machine doses more grounds per shot. A fuller basket creates better resistance.
HEADS UP Oily, dark roast beans can gunk up the shower screens much faster. Performing this brew group rinse every week or two if you use it daily will prevent most weak coffee issues. Don't use soap, just water and a brush.