Thanks for the idea. I have tried bluring the image. That is not helping.
I have attached two full examples.
The pass-page.png is an invoice with a QR code generated using ZXing in our application. That is decoded fine using the zxing decoder.
The fail-page.png is an invoice received by our customer. The QR code is not detected at all by zxing decoder. I have tested using the mobile app Barcode Scanner (by zxing). I can see on the screen that the 3 corner markers are detected (orange spot in the center of each), but the QR code as a whole is not read. The other QR code reader apps I have also fail to detect the QR code.
Using Barcode Scanner the pass-page.png QR code is detected immediately.
I can not figure out if it is something wrong with the QR code itself. Or if it is zxing that is having problems with this specific QR code.