Why doesn't my iPhone 6 scan certain QR codes?
Not all 2D codes that look like QR codes are QR codes.

QR codes have a distinctive square position pattern in one or more of the corners of the code, like this one for Wikipedia.



Based on this iPhone X teardown, it looks like Apple uses Data Matrix for 2D codes on their parts.



The iPhone camera can only scan QR codes, and not other 2D or 1D codes like UPC, Code 39, PDF417, Data Matrix, MaxiCode, etc. There are apps on the App Store that can read these codes.