No target detection at 5 star rating

Hi, I'm currently losing my mind over this. I have a database that contains 80 targets that all work fine (fast detection, stable tracking). They all have between 3 and 5 stars. Now, there's one target that has 5 stars but almost never gets detected on older devices (iPhone 5 for example), even though everything seems as though it should work. It got detected maybe twice out of 20 tests. I've attached an image of the feature points. Since it's almost impossible to change the design at this stage, I would like to know if there are any other tips & tricks I could use to solve this problem. Any information would be helpful!