I have the following two settings.
1. Settings for type 'post': Update when a post is published
2. Do not post status updates by default
On the post page "Don't Tweet this post." is checked by default. I uncheck it occasionally when I want to post but it doesn't tweet.
On the other hand, if i reverse the setting in point 2 above it will tweet and I have to check "Don't Tweet this post."
It was working fine before.