Well, this is pretty easy, and it came from this question over here:
I started searching on CSS, but after sometime I found this article:
And then I compared the v4.master with the custom master page in our project.
The big difference was only one line:
<div class="ribbon-fixedwidth s4-notdlg"></div>
After commenting this line I can see the ribbon on dialogs, Now I have other problems to solve.