Dear Francis,
It may have different reasons to happen :
1) The specification of your CDR - All CDR of different brands have different specificaiton (physically). It means there are some deviation of the thickness of the CDR and the recording materials in the middle part of the CDR.
2) The copying procedure may have problem - It is not only the copying procedure may have problems (belongs to human error) but also the technical instructment error (the burning CDR status is not stable by computer).
Suggestion :
- You may try to copy one more CDR (different brand) in order to see if the same problem happen.
- Make sure that you follow the right procedure to copy the CD.