I select all in LIbrary mode, hit "X" for reject, and
only one item takes the "X". IOW, no effect of select all.
Same for "pick", "rating", etc.
I cannot replicate your problem on my computer. Windows 10, same version of Lightroom. I suggest you close Lightroom and restart and see if that helps.
You have to be in GRID View. You can't select multiple image files in the Film Strip to Delete, reject, Star, whatever.
ok i noticed that if Library>selected button lower right, "auto sync" then
things can be done in as a selected group together. If it is only
on "sync", then things only happen one by one