Timeline Resets Itself

I have slides where I manually set the timeline to 1.5 seconds (slide shows a mouse movement/click and auto advances to next slide when timeline ends). When I preview the course or close and reopen the file, many of these slides, not all, reset themselves to 5 seconds (which is the default amount of seconds on my timeline when I create a brand new slide).

I thought setting the slide master to 1.5 seconds would help, but it doesn't. What's also odd is that as I build a scene, I copy + paste previous slides used in the scene--so why does the 1.5 second timeline stick on some slides but auto adjusts to 5 seconds on others?

I've attached two images of the timeline: one with 5 seconds on the timeline (what the slide looks like when I close/reopen the file) and the other with 1.5 seconds (what the slide looks like after I manually adjust the timeline). These screenshots are of the same slide. Unfortunately, I can't send the whole file due to confidentiality agreements with my client.

Any ideas on how I can get the timeline for all slides to stay at 1.5 seconds?