View source for Troubleshooting-Gaggia-Semi-Portafilter

#On new machines the portafilter will not fit perpendicular to the machine. This is because the grouphead gasket requires time to wear in before this can happen. As long as you can get the portafilter hand tight onto the machine and brew without leaking then the machine is operating properly. Do not use excessive force when attaching the portafilter or it can damage or break the grouphead gasket.
#Too much coffee may be dosed into the portafilter. If there is too much coffee in the portafilter is can press into the shower screen and prevent the portafilter from attaching properly. Reduce the amount of coffee in the portafilter or test the machine with an empty portafilter. If the portafilter attaches properly then the filter basket was simply overdosed.
#Check the filter baskets for dings, dents, or any sort of warping. A filter basket that is malformed can prevent the portafilter from attaching. Try attaching the portafilter with the basket removed, or with a different basket to see if it fits on correctly.
#The group gasket has slipped out of place or needs replacing. Please refer to the section of this article which discusses [[#Between Portafilter & Grouphead|gasket issues]] for instructions on how to test the gasket or replace it. 
#The grouphead may be damaged. If the machine was dropped or the grouphead was struck hard, then it can damage the grouphead. The grouphead would need to be replaced. This is not a normal occurrence and only happens if the machine has been mishandled.