pressurised filters IIRC dont require a heavy tamp.
Are your beans freshly roasted (if from the supermarket, they wont be).
Spacers for the grinder can be one solution.
Non pressurised filters another solution
Fresh Beans another solution
Proper dose another solution
Proper tamp another solution
As you can see, its not necessarily one thing that is wrong, could be a number of things, best thing is to eliminate each cause individually
First off, fresh beans, then non pressurised filter. once you have those two things then you can work on the rest, but until you have those two, its never going to good results.