1. You didn't follow up after first sale. After sale you could have introduced your other product on thank e-mail.
2. You didn't ship product in 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 an "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.