If your ad is not showing on Amazon, it could be that it has not yet been reviewed and approved after a change to the ad copy.
I know that your ad was on Amazon at one point. But if you make an editorial change to an ad that was appearing on a partner site (such as Amazon), the ad will then come down from the partner site until it is reviewed again, and approved. (More on this subject soon.)
Another thought is that since Amazon is a 'Content sites in Google's network', there is not really a way to guarantee that a particular ad will always appear there - as the content of the Amazon page (or your keyword list for that matter) may have changed.