My la Marzocco GS3 had problems with the flow from the brew head. This could be especially be seen after the shower screen was removed. The flow was non vertical with water going from left to right.
This non vertical flow can harm extractions. Water will not be pushed evenly threw the puck during an espresso pull. Opening the brew head showed clearly the cause of the problem. The gicleur was very badly aligned as can be seen on the pictures below. I used a tongue and groove pliers and some force to bend it in position. I had to be careful not to break the gicleur.
Fortunately it went well and as you can see in the video the flow is centered. It seems now much easier to get a centered extraction during an espresso pull
In case you have problems getting nice centered pulls on your GS3, I advise you to check your gicleur!