Help with beginner lesson

I've been practicing strumming and I'm a little stumped on the lesson based around "knocking on heavens door". I can't get it to sound anywhere near with the instructor is playing going on the tab that is provided. From the way it sounds in the audio there are some strums missing from the tab. Any help would be appreciated as I'm really trying to get strumming down. I can strum full cords, but when it comes to songs like this where your only hitting a few strings I'm struggling, but getting there.
Thanks,
Dan

Have you tried using the video below?
If that doesn't help, lemme know & I'll try to be more detailed.

http://www.guitartricks.com/lesson.php?input=9055

Are you listening to the Bob Dylan or the G 'N' R version of Knockin On Heavens Door as there is quite a big difference between the two, I also had this song to learn when I started taking lessons, I believe it is the G, D and C off the top of my head? It sounds pretty accurate when I play it, not perfect by any means but I played it to someone in my family and they recognised what it was.

Shcmange,
I'll look at the video this evening when I get home. I've got an early start this morning for my real job until I become a rock start LOL. Thanks.
eldante, I was listening to the audio clip that was provided with the lesson here at guitar tricks. But you are right there are several version of this song and imo all sound great. You are right about the cords though. I have the cords down, slow change between but that's me. If you listen to the audio clip for the lesson here, it sounds as though there are some stings being hit that don't seem to show up on the tab.
Anyway, thanks both for the reply and I'll report back this evening.
Dan

schmange, I didn't see the video clip before. My mistake. Thanks for the link. I'll work on this one.
Thanks,
Dan