Trying to learn how does Image Target work

Hi

I am making an AR game which is trying to turn my college campus into game stage. So I attempted to use some object like sign as AR marker.

I took photos by my phone and clipped it to make AR marker

Both AR markers got five stars at Vuforia Target Manager. The blue one works perfect, almost got recognized immediately every time. However the red one is really hard to be recognized by Vuforia. Even when I am just trying to recognize the exactly same picture I uploaded as AR marker instead of the original sign in reality.

I read the tutorial (https://library.vuforia.com/articles/Solution/Optimizing-Target-Detection-and-Tracking-Stability.html) and clipping a smaller part of the original AR marker to make a new AR marker with higher density of feature points.

It works better than original AR marker, but still hard to be recognized by Vuforia. Sometimes it took me 3-5 mins to get the real sign recognized by Vuforia. But in every test trial, once it has been recognized, it starts to work well until the app shut down or restarted. 

It is frustrated to stand in front of the sign but AR just doesn't work while it is already five stars image target.

There are three things I am confused

1.Is there anything wrong in my marker? For example, do characters and words give features but it is actually bad for recognition? 

2.Does clipping huge marker into smaller marker with better feature distributed make better image target?

3.Why can it start to work well, once it has been recognized?

Could anyone give me some suggestion? Please

Thanks in advance,

Sherlore