This is related to How can I persuade my lords to pass a law? but not a duplicate.
I am an independent grand-duke of Ireland (control 2 duchies and have no liege). I currently have minimal centralization, which is a bummer because I have no duke level vassals and a full demesne. Raising centralization from min to low would increase my demesne maximum by 1 and decrease my vassal limit from 28 to 23.
When I select the new law, it says I have 3/11 votes, all of which come from me. The other 8 mayors and bishops from my 4 counties are all opposed to the law. All of my vassals have positive opinions of me, including 3 bishops at 98~100 and a mayor at 80.
Why this isn't a duplicate: The other question is about crown authority, which I gather you can only raise if you are a king, not a petty king, so the other asker must have duke-level vassals. Is the centralization law somehow different from crown authority that prevents lowborn vassals from voting for it?
Also, the answers to the other question focus on raising the opinion of vassals. Would that do me any good if some of the vassals are already maxed on opinion? Would raising a +20 opinion to a +50 be enough to flip a vote if the +100 opinions aren't even voting for me?