Cocomo signed AIR app works on Windows but not Mac.

I have signed an Adobe AIR desktop app using a Cocomo Certificate. When I install on a Mac, the publisher is UNKNOWN. But the same .air file sent to a windows user is ok - and installs as any signed app should.

Why should this happen? A certificate working on one OS, but not the other?

How can I investigate further? - to determine the cause.

Here are some more details: The Certificate was obtained by a colleague on their PC. They converted it to a .pfx file, and sent me the .pfx, along with the password they used. I built the signed air app on my Mac.