DIAGNOSIS: You're either using too much coffee or the wrong grind size. The Stilosa's portafilter, especially the pressurized double basket, has a ridge that only holds about 14-16 grams max. Overfilling prevents the gasket from sealing.
FIX: Dose less coffee. Use a scale if you have one—aim for 14 grams for the double basket. After dosing, tamp lightly (just enough to level the grounds, don't compress with force). The portafilter should now lock in smoothly to the 6 o'clock position (straight down).
IF THAT DIDN'T WORK: Old, hardened coffee grounds are caked in the group head gasket or the locking lugs on the portafilter. Clean both thoroughly with a brush and a damp cloth. Also check that you're using the correct basket for the handle.
HEADS UP: Forcing it can strip the aluminum threads on the group head, which is a much more serious repair. If it doesn't turn smoothly, stop, remove some coffee, and try again.