It is normal to have a few positions left unfilled in a schedule. This may happen for several reasons - the most common being that the auto-scheduler is avoiding scheduling people more times than it deems reasonable. Instead of overloading somebody's schedule, the position is left unfilled so that you can choose for yourself who you'd like to fill that position.
Whenever you have an unfilled position in your schedule, you can fill it by double-clicking on it. When you double-click on an unfilled position, you will be prompted to choose a volunteer to fill the position. In the list of volunteers that appears, the reason that each volunteer was not chosen by the automated scheduler to fill the position is highlighted in red when the volunteer's name is selected. Looking at what information is highlighted in red for each volunteer is a good way to answer the question "why was this particular position left unfilled?"
If you have many unfilled positions in your schedule then there may be a configuration problem that is causing the unfilled positions. In this case you'll likely need to send us your data files by choosing "Send Data Files to Tech Support..." from the Help menu at the top of MSP's main screen so that we can look at your setup here and get back to you with an explanation and solution.