What treatment did you use and how long have you been treating the tank with it?
Whitespot can take a long time to treat and can come back after it has seemingly been cured; basically the whitespot parasite has 5 larval stages(correct me anyone if im wrong) and it is imune to meds while it is in the visable white spot stage of its growth.
The only times when the pararsite can be effectivly treated is before the white spots appear and after they have disapeared. There is not alot you can do while it is in the visable white spot stage of its growth- turning the tank temp up to 26 degrees will help this stage pass faster, only then can the meds take effect although i would still treat the fish even when the parasite is in the visable stage as there may be fish who have it but aern't showing signs yet.
Don't be too disheartened; it is a horrible desease and many people experience fish casualtys while they have it but it will get cured in its own time.
If you want to buy any replacement fish for any you might have lost, i would leave buying them for at least a month just to make sure all signs/traces of the desease has been wiped out.