Windows Explorer argument not being interpreted?

Question

I'm currently using:

Process.Start("explorer.exe", "/Select, " + fullPath); //with file name.extension

That basically tells windows explorer to open the folder where the file is in, with the given file selected.

But, by the forces of evil, sometimes it works and sometimes it just opens the file ("/select" is... ignored)

Has anyone ever experienced this?

If it matters, it's a file in a local network and the path looks like this

\\server\folder\subfolder\something\file.ext

(of course in c# every '\' is doubled, and @ is not a problem, the path is generated by something else)

I'll be constantly reading comments to supply any aditional information

--[EDIT]--

• Obviously it's not random, there may be a variant information bound to this behavior, but I haven't been able to identify it.
• Putting or not @ the beginning of the path is not an issue, since it works sometimes, then it's not a misconseption