My mom asked me to come and look at her shower. She said that for
years the water pressure upstairs has been rather weak. She and dad
preferred to shower in the shower down stairs because the pressure was
better.
She was told by a plumber years ago that she should have larger
'seats' put in, that she would get more pressure.

I wasn't aware that there were larger seats. Is this true?
I used to use that shower when I was a kid, and I don't remember the
pressure being bad. I think that there is a blockage,,, possibly a
piece of an old washer that broke loose and plugging things.

any thoughts?