As per my understanding, vortex shedding is a function of Re number. Also, from the literature it is mentioned that vortex shedding starts from Re= 40( critical Re) and in all my cases Re is higher than this value. 

If you have a better explanation as well as better understanding for this subject, I am happy with that and I will appreciate it, because my background is petroleum engineering so that I am not familiar to this subject.