Let’s see what differentiate a perfect espresso from the bad one and what are the reasons of “not expected” results.

A well-brewed espresso should have smooth layer of crema with a deep golden brown color, free from any large bubbles, pale and broken spots. The crema needs to be a couple of millimeters thick once settled, and shouldn’t dissipate too quickly. The taste should be between sweet and acidic, and the texture should be smooth and creamy, leaving you with a pleasant lingering aftertaste. The coffee should reflect its quality of itself over brewing and roasting techniques.

 What could go wrong?

If you have extracted more than 50 ml at a given time, it could be because:
The grind size is too coarse
The dose is too low

If you have extracted less than 50 ml at a given time, it could be because:
The grind is too fine
The dose is too high

If a coffee is too acidic and sour, it could be because:

The grind size is too coarse
The dose is too low
The water in the machine is too cold
The beans are too lightly roasted

If a coffee is too bitter, it could be because
The grind size is too fine
The dose is too high
The water in the machine is too hot
The beans are roasted too dark
The machine is dirty
The grinder burrs are too dull