I don't know why but the rating of my post are not showed on google search, after some investigation I found that at google master webtool site:
Reviews: Does your review use count instead of vote?
Your marked-up review uses count instead of votes. count specifies the total number of reviews for an item. votes specifies the number of people who provided a rating, with or without an accompanying review. A review can specify count or votes, or both. However, whenever you include count, the page must also contain markup for each reviewed item.
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Seems the wp-postratings stores user votes as count instead as a vote, and that's why rating are not showed on google search.
Can you modify the code to get the ratings at google search?