There's a trade-off here. If you're working in a fast-paced coffee shop, you're likely to favour the faster option (knock out the coffee) over cleaning. Generally knocking out the coffee will get rid of >99% of the grounds from the previous pull, so the impact on the flavour of the next one will be so slight as to be unnoticeable. Usually the portafilter won'...
To Expand on @Tom Medley's Answer; the pivotal questions are:
How much time is lapsing between pulls of espresso?
Are you pulling the same espresso?
The portafilter should be rinsed with the espresso machines line water after every pull. This has the effect of removing the majority of remnant oils and grounds, pre-heating the portafilter, and removing ...
In addition to Damian's answer (too much coffee or insufficient tamping), have a look at how the portafilter seals onto the group head. There should be two things you'll notice: a) a shower screen and b) threads that seal the portafilter onto a rubber washer. The following things might make the portafilter hard to turn
Shower screen is grimed up and has ...
As your question is very open I will try to answer two common roots of your problem.
1) Too much coffee. Per cup of espresso it should be 7g of ground beans.
2) No "tamping". You probably did not compress the powder in the porta filter (called tamping). This is what it looks like:
So maybe you compressed it when you tried to place it in the machine.
You probably have a broken shower (dispersion disc).
Try removing the shower screen - the metal part that faces the coffee. There's a screw in the center.
Above the screen is a plastic dispersion disc (shower); it's job is to spread the water across the bed of coffee. Remove it as well - should pry right out, no additional screws.
It's pretty common for this ...