Checking if file is open on network

[sdf.ska]

I have a simple function that returns a boolean depending on if a file
is in use on a local drive. (looking to see if a file is finished
being written to by another app) Works great from a local drive but
when I send it a unc path to the same folder, it does not work. my
question is Why? and or if anyone has a better solution to see if a
file is being used exclusively on a network share, i would be
interested to know.
Thanks
here is the function:
'-----
Private Function GetExclusiveAccess( _
ByVal theFilePath As String) As Boolean
Try
Dim strm As Stream = IO.File.Open(theFilePath, _
IO.FileMode.Append, _
IO.FileAccess.Write, _
IO.FileShare.None)
'if we succeed, we can let it go
strm.Close()
GetExclusiveAccess = True
Catch e As Exception
'Console.WriteLine("Error opening input file " & _
'theFilePath & " exception info: " & e.Message)
GetExclusiveAccess = False
End Try
End Function