Sometimes it's just a nuisance. The one I just replaced, I remember being a pain to get it.
It helps to have the machine elevated enough to be able to easily see under/inside the group. Maybe get a flashlight to have full view up inside.
Take the screen and gasket, and make sure the gasket is as far "up" the shower screen as it goes, AND, is level.
Holding the screen/gasket, lift it up to the grouphead, and push it up as far as it will go. Look up in the group. Most likely what is giving you trouble, is half of the gasket goes in, and the other half doesnt(like one side is seated, and the other half is hanging down). It made me think the gasket was to wide or something, but, it isnt. I used the blunt/flat end of a chopstick to "poke" the gasket into its spot. Whatever you use, make sure it won't pierce the gasket itself, because you may need to push a little hard.
I only use the portafilter once Ive gotten the gasket in by hand. The portafilter is just used to make sure it is 100% seated and snug. As was noted, be sure not to tighten the PF over too hard without a basket, its possible the "un-basketed" PF will "gouge" the gasket. Then, stick a basket in, and tighten it a couple times. For my machine(Anita), I can't use the blind filter basket, i need to use some type of regular basket. When Ive got the blind in any of my portafilters, it locks over a little less then If I have a regular basket in it.
Ive heard some people suggest a TOUCH of olive oil makes it easier. IDK how helpful or smart this is, but Ive never needed to do it.
I have my machine on when Im putting the gasket in. I feel that a warm gasket is easier to get into the groove.
It can be frustrating. IT may be necessary to take a 5 minute breather, more then once.
