Hola Jose,
One possibility, as Eric noted, is that you have used a faulty player ship.
Another is that you have chosen different file names from the mission title you gave in Mission Properties. The name that would appear in the game is the title, not the file name.
A third possibility is that more than one copy exists with the same name, not necessarily the same mission. They can be in different folders—one of the campaign missions might also exist in your [Users] folder.
Duplicate mission names prevent either from being accessible in-game. In that case, a red message to that effect will be shown when you start SSE, but it will be blurred for most of the time.
You can also post the files for the problem mission here, as I explained in the last post to Egas.
Regards,
Marty