Urgent Air compressor airline connection.

[mannyo]

I am mid way replacing the compressor on my 2004 D3.

I have fully removed the old one, and went to install it's replacement.

As the airline is a hard pipe, I am installing this first as I can rotate the compressor about to overcome the fact it is a hard pipe. but am at a loss now as the airline does not fully tighten down onto the compressor. The nut goes in fine, and threads all the way down but then just seems to keep spinning and not get any tighter, I put the blanking nut from the new compressor on the old compressor and it does up and locks in place.

Am I doing something wrong, is this what is supposed to happen? is it meant to tighten and rotate no more?

Help!!!!