Google Structured Data Testing Tool doesn't show stars

I don’t think the capitalize/lowercase matter, but for your case, they are other structured data as well besides rating, so I am guessing those are showing instead of the ratings.

I am not too sure whether it will show both recipe data and ratings data in Google or just 1. Not really an expert in that area.

Please see this Google search results, it should show, at least when testing the structured data.

Hmm the only thing I can thing of is the ratings name for recipe is used differently.

For the example https://www.google.com/webmasters/tools/richsnippets?q=http%3A%2F%2Fallrecipes.com%2Frecipe%2Fbanana-banana-bread%2F

type: https://schema.org/aggregaterating
property:
ratingvalue: 4.568987
reviewcount: 7881

For WP-PostRatings
type: https://schema.org/aggregaterating
property:
bestrating: 5
ratingvalue: 5
ratingcount: 2

You can try changing:
$ratings_meta .= '<meta itemprop="ratingCount" content="'.$post_ratings_users.'" />';
To:
$ratings_meta .= '<meta itemprop="reviewcount" content="'.$post_ratings_users.'" />';

in wp-postratings.php to see if it works.

If it doesn’t, you have to play around with the name since I am not an expert it this area