Simply copying and pasting the code that youtube provides into your wordpress blog will not always word.
To fix this simply take this advice:
In my particular WordPress install I can’t post YouTube videos unless I turn off my rich-text-editor on my user profile page. If you’re using that crappy rich-text-editor it will never work even if you click the little button that says, “edit HTML.” Try turning off your rich text editor, Nolawi.
Go to your admin panel and click Users. Scroll down a little bit and make sure the box next to “Use the visual rich editor when writing” is not checked.