Inserting the Portafilter
One of the causes of uneven extractions is baristas who insert the portafilter into the group head in an uncontrolled way. Your efforts to carefully distribute and evenly tamp your coffee can be undone by knocking the portafilter handle into the group head as you are attempting to insert it. The workflow of a barista is high tempo, but this aspect needs to be performed with care.
If you accidentally knock the portafilter against the group head or anything else, it is necessary to retamp the coffee to ensure it is sealed against the filter basket correctly. In this instance, you should watch the shot to ensure it flows normally. You can see a critical unevenness when it occurs. (For more details on this topic, see Lesson 3.2, ‘Channelling’.) Retamping is not ideal for espresso quality. Best practice would be to never retamp; to reduce wastage you might deem the practice necessary, however. Some cafes use separate protocols for black coffee and white coffee, e.g., for long blacks, americanos, and espressos you would never retamp.
Video showing how to insert the portafilter
For the handle to engage into the group head it needs to be inserted with the handle angled towards the left side of the machine. Imagine that when you face your machine you are facing northwards; you must orientate the handle towards the southwest to allow it to engage correctly.
The handle rotates from left to right to latch in. There is no need to force the portafilter to make a tight seal. The rubber (or sometimes silicone) gasket that the portafilter presses up against will easily form a watertight seal provided the gasket is clean of spent coffee grinds. The gasket is the rubber compression seal that helps the portafilter lock into place. (We will discuss cleaning of the group head again in Lesson 7.4.)