If you find that you lock a script and find yourself with one or more short A-Pages or Blue Pages before you've even added anything.
The likely cause is that there is something in the header that makes it too long so that it wraps and creates an extra line, which forces the material down the page and creates an A-Page.
The solution is to clear the both lines of the header -- and footer if you're using one -- and re-enter the header. That usually corrects the problem.
A big giveaway that the solution above is correct for the problem is if you move the document to a different computer with Screenwriter and the problem does not happen after you lock the script. The reason for this is that the header/footer information is stored with the app, not with the document.