It's obvious that the sampling plan alone cannot ensure that incoming lots are accepted at your facility (i.e., cannot inspect quality into a product). For example, these same lots are often sorted at the vendor site before shipment if found to violate the c=0 sampling plan to identify nonconforming items and ensure that the shipped lot is in compliance with customer requirements. Therefore, ensure that your vendor partner is measuring these critical attributes the same way they are measured at your site. Also, ensure that these specifications are well defined and understood at the vendor site. As Statistical Steven indicated, the failure of the sampling plan may be a symptom and the root cause for these failures needs to be identified. My experience is that measurement systems analysis (MSA) is one issue that often needs to be addressed so that lots released by the vendor are measured the same way as they are evaluated at the customer site.