Upload a photo or screenshot of the code. We read it in your browser, follow the link to the end, and tell you what is blocking it.
Many generators put their own link inside your code (for example qrco.de, qr.io, me-qr.com). When the trial or subscription ends, they cut that link: the image is fine, but it leads to a payment page or an error.
The target website was rebuilt, its address changed or the page was deleted. The code works, but it leads to a page that no longer exists.
Contrast too low, code too small, printed on a glossy or damaged surface, or a light code on a dark background: some phones cannot read it.
If you know the final address, create a free direct code: it goes through no one and cannot be cut off. If you want to change the link later, get an editable code for life for €2.99, no subscription. Either way, you will need to reprint one last time.