Scanning close up on ImageTargets

I have 2 trackers in my scene: 1 is 1783 x 1273 px 300dpi (151 x 108 mm) the other is 5315 x 731 px 300 dpi (450 x 62 mm)

Both have a similar design but different enough that the ARCamera can tell the difference. Both have a QR code-type design on them, both trackers are 5 star rated on the Target Manager.

When I scan just the QR code design on the 1st tracker, it will augment. However if I try to scan just the QR code design on the 2nd tracker, it doesn't nothing until you move out more.

I've tried printing out the tracker in different sizes but to no avail. I have even swapped the QR codes around and it still works on the 1st tracker but never on the 2nd.

Unfortunately I can't post examples as it's for a client.

I have tried increasing and decreasing the amount of track points, changing the proportions of the tracker by adding height and dropping in the Stones tracker in the extra space, but nothing seems to work.

The only theory I have is that the 1st tracker has a lot of concentrated points on the QR code but the rest are quite sparsely spread out whereas on the second tracker, again there are a lot on the QR code but also a lot more generally elsewhere, many of them clustered.

Do Image Targets work on percentage? So a higher percentage of tracking points are seen on the 1st tracker but a lower percentage on the 2nd, despite the very similar amount of track points?

If anyone has any idea what I'm talking about or if you've had something similar happen, please help!

Thanks