Samples are usually returned for the following reasons:

1. Sample expiration: If the sample has expired or is no longer needed, the supplier may ask that the sample be destroyed instead of returned.

2. Transaction terms: The contract signed by both parties may stipulate that the sample will not be returned.

3. Need for testing or investigation: Sometimes the supplier may ask to retain the sample for further testing or investigation.

4. Customer request: Sometimes the customer may agree not to return the sample and agree to pay the corresponding fee.

If you want to return the sample, you can communicate with the supplier or relevant party to understand their specific requirements and try to negotiate with them.