Clearcoat going 'mottled' - any fix?

[Pupp]

97 Chimaera in pearl blue (RM system apparently), finish now starting to look a bit mottled when clean. Have tried a machine polish after claying and whilst that improved it for a short while, it soon came back. Can be seen on all upward facing surfaces - a pro detailer has commented he thinks the clearcoat is failing. No lifting or flaking apparent however and I don't think it has been 'polished through' (certainly not by me anyhow).

It's never going to be a concours contender but any views on any help for it short of a re-spray?