Some MP4 videos not playable on pages/posts

Hi All,

I have 9 local MP4 (1412×1012) videos which I added by pasting the link without any formating in a WordPress page.

For a reason that I cant get to understand videos from 1 to 7 play OK and the player shows ok too.
But for videos 8 and 9 I see a grey bar on the player and if I click on the play button of these two videos I just getting kind of a buffering status, they will never be played.

I´ve double checked the links and they are correct pointing to a local server without any issues the same place the other videos are.

Any ideas what could this be?

Thanks in advance for your help and tips!