Most QR codes that "do not work" can be read after one of these fixes.

Clean the camera lens – a fingerprint blurs the code.
Change the distance: the code should fill about half of the frame.
Add light or turn on the flashlight in QRGold. In bright sun, shade the code to remove glare.
Hold still and tap the screen to focus.
Inverted codes (light code on a dark background): QRGold reads them; some camera apps do not.
Damaged or very small codes: take a sharp photo and scan the photo instead.
Turn on the right format in QRGold settings if it is a barcode (EAN, Code 128, Data Matrix…).
Scan from a screenshot if the code is on your own screen.

Scans but nothing opens? Some codes contain only text, or a link that has expired. QRGold shows the content so you can see what is inside.