Treatment for whitespot has to to be repeated several times. The parasites can best be treated when in the free swimming stage, so repeats are necessary. Intervals between treatments depends on the water temperature.
Fish are always more susceptible to ailments when stressed, such as a transfer to new aquarium or too big a water change.
Some dealers run water from one tank into the next on etc. so filtering many tanks with one system This also means of course, that any free swimming pathogen can easily infect many tanks, so always look at the fish in several tanks where you are buying from.
At the free swimming stage you probably won't be able to detect parasites so even when the fish look healthy, it is not always so.