Bad OT feature behaviour in Quark 7 & 8


A client of mine is reporting some strange behaviour of the OT features of a font in Quark XPress 7 and 8. In some documents the features are accessible, in others they are not. The OT features of this font work properly in Adobe InDesign.
Is this a Quark issue? If so, what is the best work-around to avoid this behaviour? It also seems that the features work in the same document the other day when they were not working the day before. ?
Any advice greatly appreciated.