Quality Assessment of a batch of file

Hallo to all,

I hope you can help me. Yesterday I sent the return package of a project containing 3 bilingual files (EN-IT) to a customer. I had carried out the QA procedure and the final report didn't show any mistake, but when  the customer has repeated the QA some inconsistencies have appeared. I've repeated the procedure but the final report still doesn't show that inconsistencies. How can I solve this problem?

Thank you in advance

Daniela