An analogy is if you have leaks from your ceiling when it rains, and you kept patching the ceiling without first repairing the source at the roof, you wouldn’t be surprised to find your ceiling kept springing leaks every time it rained. Likewise, with sclerotherapy, it is critical to treat the feeder veins (reticular veins) before treating the spider veins. If this is not done then just treating spider veins will at BEST just give a temporary result and more often make things worse.
Doctors who have not gone through a proper training program will unfortunately often just treat spider veins without first treating the reticular veins.