How to respond customer complaint regarding pH out of spec

Hi all, 

Our product is edible  ice, customer had complained product pH out of spec. 

For all information, our raw material is  merely water only. So the pH content highly dependent many factors, such as temperature, and the way that lab handling our product during testing such as sample leave at room temperature to long , etc. 

In such case,  any idea how do we respond to customer, especially 'root cause' part. because this issue as if "beyond our control" it may reoccurrence in future. 

What is the range on your spec and what is customer claiming pH was?

Do you perform any testing on your water? Do you get an annual notice from the municipality providing your waters, specifying the pH range?

Kindly share the exact customer complaint. It will ease to respond.