As Travis has mentioned, there can be alot of different issues affecting one's affiliate sales.
If you're using affiliate networks such as Commission Junction, Share-A-Sale, NetShops etc., one has to make sure that you keep up with their newsletters and other affilate communications so that you know when things have changed, are discontinued or other "need to know" issues that may affect your promotions.
Keep track of your stats in your aff network's back office.
PLUS you need to keep good records on "what" you're promoting and "where" you are promoting them. I'm a bit lax in that regard, I hate to say, but I seem to get it working anyway!
Hope I made some sense. heh heh
