Hello Donna,
Sorry to hear that you keep running into this behavior.
Whenever slides that are imported come up as blank, this usually indicates the presences of corrupted data. In some cases, the entire slide is corrupted, and in some, the presence of a single slide element or object is what's causing the slide to appear as blank in Story View.
A common cause of this is working on a network drive when making edits to a project file. The introduction of network latency usually causes files to misbehave. If you're importing older files that were created in Storyline 1, 2, or 3, then it's also possible that formatting could be what's causing the behavior. We recommend working working with our support team by opening a case here so we can identify the root cause of the issue, but I'll also be glad to take a look at some of your affected project files if you're comfortable sharing them here!