iTunes doesn't play movies, it shows the image always in pause

Hi, I have a MacBook Air (2012), and I have converted a few mkv. films into m4v. with MP4tools. The problem is that, even though Quicktime plays them, when I export them to itunes, and I try to see them, the image is always in pause, it displays the image of that second, but it never reproduces it.

I don't know what to do, and the files are correct, because if they were not, Quicktime wouldn't play them.

Please, help me, and thanks.