Why I can't edit some page's content?!

Hi there. I’m a web designer, so yes, I can design and html+css.
But I’m new at WP, so here it is:
IN a Template with 8 pages pre-maded, I can’t edit some of them, cause they simply appears as “empty” in the html/Visual Field.
I go on All Pages. Then I click on Edit at “About us”, for example.
And Voi-la: The html content or Visual content are there, Super!

But if I go to Gallery, p.example (which is a page that yes, it exists!) , the HTML/Visual Field is completey blank.

What’s wrong whit it?!
Thanks!

Paulo