FindInFiles problem pattern matching

I am trying to process all the files in a directory with a particular extension.  I am using the following line of code, but it is returning nothing in strMarkerFile

For Each strMarkerFile In My.Computer.FileSystem.FindInFiles(My.Settings.printspoolpath, ".txt", False, FileIO.SearchOption.SearchTopLevelOnly)

The printspoolpath string is a network path representng a samba share on a linux server.  By this point, the path has passed a DirectoryExists test and if I give my file handler the full path to a known file in the directory, it reads it fine.

Can anyone give me a tip

thanks

Gareth