Might be better to change the treatment....try another brand of whitespot treatment.
WS is generally caused by stress.....either poor water conditions, poor food (causing a poor imune system), or the fish being moved from tank to tank a lot.
Feed the fish well....little and often is always a good idea and try some different food eg if you're feeding flake, try adding some small, high protein pellets a couple of times a week and maybe a couple of cubes of frozen bloodworm (defrosted first).
Good luck