Unfortunately I think you stumbled upon a type of image that does not make a good canidate for tracking.  Two things might be going on, the line features of it are overpowering the corner features and/or Vuforia might see it as being repetitive, even though it doesnt look like it technically.  

Maybe read this guide if you haven't: https://library.vuforia.com/content/vuforia-library/en/articles/Solution/Optimizing-Target-Detection-and-Tracking-Stability.html

This marker generator produces good result if you just need some random geometric shapes.http://www.brosvision.com/ar-marker-generator/