Where you went wrong...
First, in response to the author's comments, what do you think we are, two?
Secondly, there was no animation apart from tweening in that entire flash. The cow's lips don't even move.
Third, the joke was fairly predictable, and your attempt to make it funny by drawing out the wait for any response fell flat on its face. I honestly don't know if anybody else is going to patient enough to watch to the end of the flash like I did.
One tip you may find useful for your next submission is to keep your audience in mind. An animated version of a knock-knock joke is not going to appeal to a 13+ audience.