I've noticed some bugginess with Publicize not working at certain times for a little while now. After trying out a lot of different scenarios, I think I've narrowed the issue down to this.
It seems that when you're editing/writing one post and a different scheduled post goes out, Publicize does not fire.
This happened to me this morning. I had a post scheduled for 10:24 and at that time was working on another post, the time came and went and by 11:00, it still hadn't shown up on the selected social networks, so I went and manually added them.