Not understanding it. Why does your product have a request to be returned? They have no idea where to begin with using it so they do not find the value in keeping it.
In the world of client issues with the your product a majority can be aided with a simple fix: education.
Being the expert in your field of product or service gives you the advantage over others. The client came to see you for a particular reason. Give them that justification in seeing you by showing them they are correct in that assumption. Discover what your client expects from a product based on their needs then make the recommendation to give them the desired outcome. If your client has a predetermined idea for what you can provide discover why they feel that way and either reinforce their choice or steer them in the correct direction for what will meet their needs. Explain the why of each situation reinforce the outcome you are presenting to them based on their needs from you the expert.
Educate your client. I imagine area you work there is an understanding of it that is possessed beyond an outsider. Leverage this to your advantage simply by explaining inner workings, that is permitted, to them. Why your organization does the things it does. How the science behind a product works. What does it affect in their daily lives to be a benefit to them. Sit down with them on a personal level to discuss their concerns they have to work through them. Answer the questions that they have with confidence.
As time passes the product you worked together on before has now has most likely taken on a new meaning in their life. Give the client resources that will benefit them when they advance to a future stage then they are right now. Do not be afraid to allow the customer to reach out to you again for questions. If possible direct them on how they can get questions answered easily in online channels from the comfort of their own homes.
A satisfied client will become a loyal client for the future. Ensuring they will become loyal is to educate them at the beginning with giving them a security net of education to better understand what you have given them.