From a reply standpoint, I'd start by asking for a copy of the COA to review the testing method (as eluded to above, the test may have a margin of error you're falling into). The COA should also indicate the temp of the sample when it was received, and just from a quick google it appears that ph of water fluctuates at different temperatures (meaning if they let it thaw into water and sent it in a warm truck, the ph could've dropped below that spec).
If you've got a retain from that lot in one of your freezers, I'd send it in for independent analysis on your own. And I'd also send a batch of current ice you're producing for testing as well to help determine if you've got an issue with the incoming water you're using (which I assume would be city water, let me know if that's not accurate). I'd also have the lab test for chemical additives that the city uses (should be on their water report) to make sure the additives are all within spec.
My response would then depend on what is discovered through your investigation. Could range from "you mucked up the test" to "we're unable to duplicate your result" to "we have isolated a variable causing the discrepancy and performed a corrective action."