I am trying to raise crown authority, but currently too few lords support the vote. Are there any methods to persuade the lords of the realm to support the new law?
When you hover the mouse over the vote count, it'll tell you who's opposing (more like, not voting for) the vote. Not all voters have the same weight, some count more than others and you can see that here. Concentrate on the bigger voters and those who have a least negative attitute towards you. If the opinion is not too negative, a simple honorary title could make them vote for you (+10/+15). After that, try having them educate your child (+20). If money is not an issue, a gift I think adds +20. Or if the vote is important for you and there's a voter that has a heavy vote, you might even try giving them a land. Also maybe they have a claim on a title, well, if you press that claim it's an automatic +100. Another thing you can do is select him/her to your council. Check their current ambition, if it's getting married, arranging that marriage I think adds relation to you too.
Raise their opinion of you. Give them titles, send gifts, be a ruler with many virtues. If they like you they would vote almost immediately.
As well as the obvious Honorary Titles (DON'T GIVE COURT JESTER), Education, Gifting, granting a landed title, and pressing claims, try making it faster by having a Feast every 2 years.