Hi the key to master/sub projects is to ensure file names and locations dont change. This is clearly impractical for most users.
A better way is to create the master program when you need it by using the window/new window command (view, new window in 2010). To do this open all your schedules, then use the new window command and select the view (from global) you would like to use. As an aside you can adopt all sorts of custom views to do some funky reporting.
You then format, filter etc until you get the display you want. You can then save the master.
Next month do a similar process with the updated files.