For some reason wordpress gets a bit finicky when you try adding YouTube to wordpress. Try adding the YouTube code this way :
Create a new post or page where you want the YouTube clip to show. It normally opens in Visual ( top right in the box where you add content) click HTML instead and place your code. Dont press preview but publish instead. Voila YouTube clip should show up.
For some reason wordpress has a bit of a hissy fit when you try preview first and doesn’t show the clip but a grey box instead.