First, if you're posting to Twitter make sure the video is the appropriate length allowed by Twitter. If too long it will not post correctly.
For Facebook, make sure the video is not violating any creative commons licenses. If it is ie: using someone elses music the video will simply not post. Note that even though SociVideo Jukebox is using YouTube creative commons search... there are a number videos Facebook will still not allow to be posted natively from that search engine.
Next, make sure you leave your computer running and have a strong internet connection. The worse your internet connection the longer it will take for the videos to post.
If the video is large, please note that it will not appear on your Facebook page at the time it's scheduled. There may be a delay depending on your bandwidth when the video actually is posted.
If you're going to post a large video, try scheduling several minutes ahead of when it should be posted to your Facebook.