How can I raise the Centralization level of my Petty Kingdom?

When you propose a law, all your vassals are against it at first, but they can change their opinion over time and change their votes to "Yes". Vassals are more likely to change when they have a positive opinion about you and when the law benefits them personally (unlikely to be a factor with centralization, but can be very important with succession laws, for example).

Just propose the law and wait for a few months. You will see that some vassals will start supporting your law. When you have some powerful vassals who refuse to back it, see if you can improve your relation with them somehow.

Sure enough, the law was passed after a few months. I had been toggling the law vote on and off, experimenting with different laws etc. trying to understand what was going on. I was under the impression that the law votes were like the character-level interactions (e.g. marriage, invite to court) and they wouldn't change on their own without an outside influence.