MetaWeblog API

We have a remote blogging application that is used to edit posts in WordPress. For some reason, the MetaWeblog API won’t connect to some setups, configurations or versions.

I have searched far and wide for a common thread, but can’t find one. It doesn’t seem to be related to plugins or versions or the XMLRPC.php file.

Things will look exactly the same on a blog where it will work and one where it won’t. I know this a very general question, but does anybody know of any problems here? Any issues that would cause the API not to connect to some blogs? Thanks for your help.