The Group Head Is Blocked
The group head is one of the most often overlooked parts of an espresso machine, which is why a blocked group head is one of the most common causes of slow coffee dispersion. Every time you brew an espresso, the coffee leaves residue on the group head that can stop water from getting through at the right pace. To fix or avoid a blocked group head, make sure you clean it regularly.
You're Putting Too Much Coffee In
If you overfill your machine with coffee, the water will take longer than it should to dispense through the grounds. On top of that, you'll also be wasting coffee grounds that the water can't absorb (and, therefore, wasting money). When brewing, always check that you have the correct filter basket in your filter holder. If you're brewing one measure of coffee, use the single filter basket and holder. For two measures, use the double basket and holder.
The Operating Pump Pressure Is Wrong
Since coffee grounds must be tightly compressed, your machine needs to force water through them with a high degree of pressure. If your operating pump pressure is set too low, your coffee will dispense too slowly—or sometimes it won't dispense at all. The optimal pressure is considered to be around 7 to 9 bars, so make sure you check the needle on your gauge before brewing to see if the setting is right.
You're Grinding the Coffee Too Fine
If you grind your coffee too fine, the particles will fit too tightly together and prevent water from getting through at an adequate speed. Thankfully, changing the grind is easy. Simply rotate the collar on your coffee grinder until it's at a coarser setting. There may be some trial and error involved in getting the grind texture right, so make sure you pull a test shot of espresso after each change to see how fast the coffee is dispensing.