Check to make sure the AutoPreview box is checked.
- When you click the Play button at the bottom of the Custom Animation task pane, this single slide will play in the current window, showing any animations applied to the slide.
- When you click the Slide Show button at the bottom of the Custom Animation task pane, the
slide show will play in full screen, starting from this current slide.
- Click the Slide Show tab on the ribbon and make a choice, or press the F5 key on the keyboard to play the complete show in full screen.
Complete 10 Part Tutorial Series - Beginner's Guide to PowerPoint 2007