serene video embeding

I hope someone can help me fix this.
I’m using Serene Theme (Version: 1.4) and when trying to embed a video so it displays it on the header of the post sometimes it works and sometimes it doesn’t (??)
First it took me a while to figure out that the way to embed the videos was through the old wordpress embeding code:

[youtube https://www.youtube.com/watch?v=8i8Vb3c3rc4&w=1280&h=540&rel=0%5D

because if I try it with the new embeding code:

<iframe width=”1280″ height=”540″ src=”https://www.youtube-nocookie.com/embed/1aGF1aNd5eo?rel=0&#8243; frameborder=”0″ allowfullscreen></iframe>

it doesn’t work.

Now the funny thing is that it only works in some posts not in all of them.
Here is an example of it working:

https://blog.lorenzobennassar.com/simurg

And here is an example of it not working:

https://blog.lorenzobennassar.com/forstarters

and the way I embeded the video is exactly the same (??)

Any one can help me with this?
Thank you so much