Is purpura the guarantee that Vbeam laser is working?

And if the skin doesn't turn purple, it means is not working and I won't see results? I have been going for Vbeam sessions for the past 4 years, for a pws mark, and sometimes the purple dots don't appear. From what I've noticed, if they don't appear it means the laser didn't work and I won't see results. I feel like my doctor is trying to keep me longer as a patient, by doing this. So, my question is, is purpura a sure guarantee that the laser works? Thank you