I'm having the same problem here. Hit or miss posting behaviour.
On my Publicize I have connected a Facebook and Twitter account, not without any trouble, had to retry several times for my social accounts to get connected.
Once they were connected, made a test post that was posted on both social sites with success. After that, made something like 15 posts with Publicize enabled, directly published and only 4 were published on Facebook and Twitter.
So has idoubtit said this is VERY hit or miss. Sometimes it work, sometimes it fails completely.
Could someone point to plugins alternatives for automate posting to social sites, mainly Facebook and Twitter?