When you copy & paste text from other posts or web pages, you can pick up bits of code (which doesn't show up in the message editor) that can create unexpected effects in your post, or cause a URL link to not work correctly.
If I copy the video URL from the above post, paste it into my message and then put the code '[View: :550:0]' around it to make the video appear, it will work.
If I type the code first, then paste in the URL, it does not work, but in each case, before I post the message the text looks exactly the same.
It's only when you go into the HTML editor, you see that copying the video URL and pasting it has brought with it some extra code, and it is the position of that code that stops the video from appearing, and that code is only visible in the HTML editor.
There is a trick you can use when pasting text from other sources which strips out any formatting, and that is CTRL + SHIFT + V which should help to minimise some of the problems of this nature.
⇩ EDIT : Thanks tigre. ⇩