Maybe you’re looking in the wrong place for the value.
The Streaming API is in real time. When tweets are created and streamed, their retweet_count is always zero.
The only time you’ll see a non-zero retweet_count in the Streaming API is for when you’re streamed a tweet that represents a retweet. Those tweets have a child node called “retweeted_status” that contains the original tweet that was retweeted embedded within it. The retweet_count value attached to that node represents, roughly, the number of times that original tweet has been retweeted as of some time near when you were streamed the tweet.
Retweets themselves are currently not retweetable, so should not have a non-zero retweet_count.