When it exports, is it exporting the subkeys below this? The New Signature key and the Reply-Forward Signature are found in a subkey.
Another question is, in your PRF have you set it to replace the profile if it exists. If so than it will be overwriting your settings.
Lastly are you sure that the entire key is identical on the new machine, so for example the "Default Outlook Profile" isn't named something different?
Cheers