The problem for inter milan and napoli is money, they not have a lot money to buy or if looking into pogba story, why pogba want to joined to juventus because he got one rarely super car all is about money if a team don't have a lot money is impossible to be the winner with the exception of leicester is happened once in every 40-100 year.
Napoli is quite rich imo, I mean, how can a poor club bought Higuain with 40 millions euro from Real Madrid and brought another good players there like Reina and Callejon. And Inter, I guess they got pretty much money when they won treble winners. But seems they made bad management after Mourinho leaved and now they are suffering. So i guess the main problem of those clubs is not money, it just because of unstable performance and bad management.
the president of napoli is rich, but if like win the italian serie a, request a really big investment, and i think De Laurentis not like spend 100-150 millions euro ( i think sell Higuain this year with big offert).
Inter, Thoir for now speack more but i don't have see all this money.
For Milan, Berlusconi don't like more have the propriety, not is money problem