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I have an idea why the tag detection is not working as well as it could be.

The white border areas around the tag is very important for it to be detected robustly, ideally you want this to be at least 2 squares of the pattern wide. Also if you could make sure the tag and the border are kept as flat as possible this will help to remove inconsistent shadowing artefacts.

Your second image has pretty much no white border at all, so I wouldn't expect that to be detected very well at all.

Hopefully this helps you get this working reliably.