What version of OpenLP are you using?
In my testing, I can't reproduce any problems. However, PowerPoint itself has an issue when running presentations in fullscreen mode. If the fullscreen window had focus at one point and then lost it, it pauses all animations and slide advancement.
I will add a fix to resume the presentation if it gets paused. If your issue is different from what I described, please let me know so I can address it properly.
