Pythons, especially of the Morelia genus, will start ugly, get nicer, then start to slowly dull in their intensity. I think that is just how it is. It's not neccissarily a bad thing. Sometimes you'll have one that stays nice all through adulthood, but this is generally an exception to the rule.
I have talked to a friend who has been to your country many times and has seen many Morelia in nature and says they are all darker and uglier than the ones we have here in captivity. He says they just get dull as they age.
How many have noticed such a reduction in intensity in their carpets?