1. You didn't follow up after the first sale. After the sale you could have introduced your other product on the thank e-mail.
2. You didn't ship the product in the about of time you stated. If they needed it in a hurry and you didn't provide, they won't rely on you again.
3. Your product didn't do as promised. If your product didn't accomplish their desired goal they're not going to think your second product will either.
4. Your customer couldn't get a hold of you in time when they had a "after question" sale. You could have added extra lines of communication.
5. Your customer doesn't want to revisit your web site because it didn't offer much. You could have offered more original content or freebies.
6. Your competition is offering free shipping with their product. You should have been more aware of how they are targeting your customers.
7. Your customer forgot your web site address. You should have given your customers your web site information in your product package.
8. Your customer service couldn't solve a problem they had with your product. Your customer service should be trained to handle most problems.
9. You didn't up-sell when they were already in the buying mood. You can always try to sell your other product when they're ready to buy your first one.
10. Your competition offers a stronger money back guarantee. You must always be thinking of better ways to remove the risk from your customers.
About the author:
Wesley Atkins is the owner of http://www.reviewbooth.com/ - containing numerous web site marketing tips, articles, and reviews of marketing tools and software.