If you want to insert slides from a template or another project, it's necessary for both Story files to have the same aspect ratio. You can adjust this setting under the "Design" tab, in the "Story Size" section.
If the two files have different aspect ratios (e.g., 3:4 and 16:9), the objects may not be placed correctly as they are tied to a sort of coordinate system in the backend.
Occasionally, there may also be display issues in projects with the same aspect ratio. Here's an example:
Let's say your template is set to an HD resolution (1920x1080px), and the headings are defined to look good with a font size of 54pt. Now, if you copy this slide into a project with only 720x405px (the default 16:9 aspect ratio in Storyline), the text may not automatically scale and will retain the overly large font size of 54pt. By adjusting the resolution to match, you should avoid any further display problems.
