Replace with string not working

I have this line of code that is suddenly not working.
fa is a folderitem that is inside/under thcoll.

Dim fs As String = Replace(Fa.NativePath, IniHerd.ThColl.NativePath, "")

It used to remove that portion of thcoll from fa. Now it just leaves it.
If it could be OS it is Mac. Current XOJO

It would greatly help to know the actual content of fa.nativepath and IniHerd.ThColl.NativePath.

FA.NATIVEPATH cannot be changed… it is READONLY

Look at PARENT and CHILD properties

Sorry Michel. I had the wrong property