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.
- Choose Slide Show > View Show from the menu or press the F5 key to play the complete show in full screen.
After viewing the slide show, you can make any necessary adjustments and preview once again.
See Related Article - Apply Animation Schemes to PowerPoint Presentations