In my experience the only staining on UPVC that you cannot remove with WFP is the reminants of traditional cleaning where the sills have not been cleaned off properley and the soap/detergent has stained them. There is no logical explanation for WFP pole to stain the sills and not the rest of the frames. Infact to go further, i have cleaned several with staining on the concrete lintels and the marks on them are terrible, its almost exclusivley the result of soap suds loaded with dirt running and drying on them.