There is a new strain of ich that is resistant to the average treatments.
At work we have some platies that have been treated with three different courses of whitespot treatments and none have touched it.
Two days ago I started to use Cuprazin® by waterlife products(tm), which is a copper based medication and most of the platies seem to be improving.
I will say that you have to be extreamly careful of using any copper based meds as an overdose will quickly kill all fish and it should not be used in tanks that contain inverts etc.
The dose I am using at work is 1ml per 30 ltr and the course is for 5 consecutive days.
HTH