Diagnosis
You're not creating sufficient resistance in the portafilter. The two main culprits are grind size (too coarse) and tamping pressure (too light).
How to Fix It
Grind finer. This is almost always the solution. Your coffee grounds need to be fine enough to create the 9 bars of pressure the machine is designed for.
- Adjust your grinder to a significantly finer setting. For espresso, the grounds should feel like fine table salt, not sand.
- Dose correctly. Use the double basket and aim for 16-18 grams of coffee.
- Tamp firmly and evenly. Apply about 30 pounds of pressure—enough to fully compress the puck.
If That Didn't Work
The machine's over-pressure valve (OPV) might be set too low from the factory, dumping pressure early. This is a common mod. You can adjust it with a 8mm hex key and a pressure gauge, but it requires opening the machine. Search for "Gaggia Classic Pro OPV adjustment."
Heads Up
Don't use pre-ground coffee or pods meant for pressurized baskets if you're using the standard (non-pressurized) double basket. They won't create enough resistance. If you must use them, switch to the single or double pressurized basket that came with the machine.