This happens when you add measures. The automatic setting is to stop at the number of measures originally selected when creating the document. (The default, if I remember correctly, is 31 measures.) As an automatic setting, this makes absolutely no sense. Many of us start with the default number of measures, knowing that the additional measures will add as we go, or can be added in bulk under the Edit menu. Then we try playback and it stops after a certain number of measures. Only if we know about this automatic setting will we know to change it as we go. The automatic setting should be to play to the end, which would save us all a lot of time and confusion.