My case is opposite to the one post in here. I have used the 40 max votes yesterday and until now I have not received the Vox Populi badge. Are there any rules on how to distribute our votes per day? May I know some explanation? Thanks. I don't know if its proper to tag this as bug.
In the interest of having an answer, lifted from a comment.
Yes, if you want to cast 40 votes a day you need to distribute them correctly, specifically you need to vote on questions enough. Otherwise, you max out somewhere between 30 and 40, and it appears this is what happened. For 30 votes one gets the Suffrage badge, which you have.
In particular, it is not necessarily enough to vote until one gets told one cannot vote anymore. Some more details are on Why can I not use my additional 10 votes on just questions.
Possibly this is vaguely a duplicate of this report.