The trick with YouTube links is to only have the bare minimum part of the link.
Sometimes YouTube links look like this:
Everything after and including the "&" needs to be removed. All we need are the numbers and letters after the "=" sign that is really part of the link.youtube.com/watch?v=-r-UtIffUnw&feature=feedrec_grec_index
So it should look always like this:
Also, sometimes the text of your link does this and it won't work until you fix it and put all of it on the one line:
There is also the recent problem of Youtube using secure http, so if the URL is otherwise fine and looks like this:
What you need to do is edit and remove the "s" in https. Forum software all over needs to sort this out, but until then, it needs to be done manually.
A good idea when posting tricky things is to use the preview button and see how it all looks.
Once posted there is also sometime to edit as well.
Once done correctly it will appear like this:
In addition, if you try to use a mobile link then it won't work either, you need to full web link. To fix those I do a new search on the PC for the same video and then use that link to update the mobile one.