Good question. 
I’ve made a similar comment in another thread. Some dreamsigns are very common in my dreams, but they don’t trigger lucidity. Other dreamsigns are rare, but they very often trigger lucidity. What to do then ?
First, to have a look at these two different topics…
Dreamsigns
Dreamsigns
It seems that effective dreamsigns (I mean DS’s which trigger lucidity almost automatically) are somewhat similar. It’s often something which is related to focusing attention, like flying (I have to concentrate), looking in a mirror, be chased, fighting, etc.
It’s my opinion, I’m perhaps wrong.
I’m under the impression that, in order to transform a non-effective DS in an effective one, a specific effort has to be done. Perhaps you could try the Laberge’s method (described in MILD) : you remember a recent dream, visualize it and when you meet a DS, imagine that it triggers lucidity (“Waw ! I’m dreaming!”) and imagine that you make a RC and do what you planed to do.