Unix copy method not working for gnome terminal

[oxy]

>- highlight to copy, middle-click to paste
>- highlight then edit->copy (or right-click->copy) then ctrl-shift-ins
 > or edit->paste or right-click->paste.

>Which one doesn't work, are there any circumstances under which it
>does work?

highlight to copy doesnt work!

I open a file with vi, highlight-copy, open a 2nd term and middle-click.
A wrong text comes from an old memory.
I leave vi and `cat filename` and highlight-copy the same content and
press middle-click on the 2nd term, it works.

Sth seems to make no sense here. For some reason vi is hindering copy.