My new portafilter doesn’t fit, even though it is listed as compatible.

The reason for this is related in almost every case to the old portafilter and the portafilter gasket. Because the ears of older portafilters are worn down, which is compensated with a thicker portafilter gasket. Because of the thicker portafilter gasket the new portafilter won’t work. So it is handy to also order new portafilters with your portafilter to prevent this issue.