Sofa cushions

Mine do the same - the cushions on the old suite never did this (same make, Gplan). It's annoying. They were a different shape, though and much better.

I just move them around, turn them over, swap them around, the covers move back again.
If they're the same shape and size that should work.

My local upholsterer has managed to make my cushions do the annoying "slipping about" thing. He is well respected and very experienced and this is the fourth time the sofa has been reupholstered since it's purchase second hand in 1950. It is the FIRST time the fabric has moved about like this.
I think it's the make-up of the fabric. Too much viscose probably which is very slippy.

Thank you all. I’m going to look for an upholsterer, see if they could be made firmer too.

I am sure a local upholsterer would remake them for you at a fraction of the cost of replacing.
You could also have the filling of your choice.

I’ve just received a huge bag of stuffing ( fire retardant ) from Amazon to restuff our cushions, not easy as the stuffing is in little compartments, but we managed, but trying to get the covers back on was a trial and took 3 of us.
But we now have comfortable cushion seats, we were thinking of getting rid of the sofa it was so uncomfortable.

could you take/post a photo?
are they stuffed with poly? I'm not sure I quite understand