A family friend will help me troubleshoot my electronics. So, I have seen the other side of this: I get something which is completely nonfunctional and send it back to the manufacturer who claims it is functional, and tries to bill me for the shipping both ways. That's why if I can not get a working replacement in 2wk or so I just send it back for refund.
For a real example: Are you using very well made power supplies? Maybe your board's power filtering could be better, and customers notice because they buy the cheapest supply that meets their power requirements.
In general we can diagnose and repair or replace the systems rapidly. This one case was an extreme outlier, and in the future we will just be authorizing RMA and replacement / repair.
The trick is to get your board manufacturer to eat the return cost. If you can buy testing, do so; usually it comes with some assurance that the boards work, and they either make new stock or remanufacture what you send back. If you can point to a comprehensive testing instructions that the customer would have performed it should not matter if you receive the board and it works for you, for some reason it did not work for the customer.