This has always bugged me and I decided to try and find out why it happens...

Sometimes but not always, when I have selected one or more paragraphs and
drag one of the indentation sliders on the ruler (like First Line), nothing
happens to the selected paragraphs. For the moment, the slider is located
wherever I put it but when I click elsewhere (or reselect the same lines), it
goes back to its original position.

Why does this happen? Does it have to do with what was selected or how it
was selected? It's pretty frustrating...

THANKS!