You didn't Clear Cookies
Remember you HAVE to clear your cookies often or offers simply CAN'T approve.
You used an Email previously used on that offer group
You can only use an offer ONCE on the same offer group. Don't know if you've used a certain email before? Just use a brand new email!
You submitted FAKE information
Fake information is NEVER allowed and will lead to your account being closed, but before that offers simply deny your lead/sign up.

You didn't fully complete the offer/survey
You MUST fully complete the offer, this can be from either:
a) You can go no further
b) Reached a congrats page
c) Confirmed email(s)

You didn't "mark as complete"
Are you sure you marked as complete, because offers CAN'T approve unless you do...
The Offer was inactive so you got redirected
If an offer is old or inactive then the Advertiser may decide to redirct the link, however the tracking cookies are not always redirected which means that if you complete the offer you MAY not be approved.

And finally...... You didn't wait long enough!
Offers can take anywhere from several minutes to several days to approve. If it was a paid offers then it could take several MONTHS to approve. the point is, don't worry most offers will approve if done correctly!

The point of this post is to point out the main reasons for you not being approved, but at the same time it is possible that high approving offers dont approve even when done right, this I cannot explain, maybe the advertiser did an update whilst you were doing the offer which cause the tracking cookie to stop working, who knows?

But please be patient and don't moan when your offers don't approve NO ONE has a perfect 100% approval rate (for those with a lot of completed offers anyway).