You may notice that there are two targets listed for FrontPage RPC:

Some commands require that you post to the _vti_bin/_vti_adm/admin.dll instead of _vti_bin/_vti_aut/author.dll.  How do you know which commands should talk to which dll?  For one, the SDK is pretty good about telling you which one - check the Decoded section and you should see something like this:

Otherwise, you can always try it against the author.dll and switch to admin if you receive a response like this: