If this is only effecting one slide, it may be possible that the mouse click transition may be disabled for the slide. Please pull the slide up in powerpoint's edit mode and click on the Transitions menu bar at the top. You will see the option on the far right under "Advance Slide" and make sure the option "One Mouse click" is enabled for that slide.
"On Mouse Click" rather (typo)
Many thanks Handy Dan217. Problem is solved.