I would not authorize a return until I had a complete understanding of the problem. Samples, run conditions, specific evidence (prior successes and problem material samples) that the problem was created by my compound, etc. should be offered by the complaining customer to you for your analysis. A visit to the customer may be in order, as well.
There would have to be clear evidence, presented by the customer and varified by myself or my technical staff, that the problem was clearly due to my compound before I’d authorize a return or credit.
I'd review all my documentation regarding the lot processing, raw materials, and any internal QC checks on the compound, before contacting the customer.