git bug

someting is not logical in the behaviour of git :

suppose I have a project and a file in it I don't want to include initially in.
so I put this file name in .gitignore
now all is OK when I write "git status" : the file is ignored.

but later I want this one be a part of my project.
I delete in .gitignore the line that named this file.
but now a "git status" command ignore always this file.

what is wrong ?
did I missed something ?

thanks for any answer