In most cases it is due to the PC having to little RAM and or too slow a processor. Can you give some details of your PC/MAC. A good tool to use is Speccy which when you run it will show much about your PC. Please note the minimum PC resources quoted by Magix are not anywhere near enough to run Magix PhotoStory smoothly.

My last PC far exceeded the requirements but it took ages to load Photostory and editing was slow jerky with the program often crashing.