I don’t know if this is relevant, but I found that on my blog the yoast_schedule_tweetbacks() function kept picking up the ID of the last entry in the ‘Related Posts’ widget, instead of the actual post being viewed. So, every time the plugin was looking for tweetbacks, it was looking for the wrong post.
Adding wp_reset_query() to the beginning of that function seems to fix it. Removing the sidebar widget would probably have the same effect, though it may depend on your theme too.