Documentation states: https://helpx.adobe.com/in/captivate/using/master-slides.html
Main master slide
The main master slide is the first slide in the Master Slide panel. The name of this slide is the same as the name of the theme. Objects and background color of this master slide appear on all other master slides. Each theme has one main master slide.
It is not true and leads to confusion. I've confirmed this with discussions of official Adobe Captivate support agents. Some master slides cover content, like images or text objects that are on the Main Master. There should be a statement like this:
Objects and background color of this master slide appear on MOST other master slides. Some other master slides may not reveal objects on the Main Master.
If I am wrong about this, please correct me and I will be deeply ashamed and embarrassed and I will soak my head in a bucket of lukewarm water. (Hey, I don't want to hurt myself.)
There is a lot confusing in the Help, I am telling that since 10 years. I have been moderating and correcting the Help, adding links several years ago, but that inititative is now closed without any improving of the Help documentation, on the contrary.
The Main master slide can indeed transfer its color, and objects to other master slides:
I don't know which version you are using. The themes included with Captivate are not all respecting the master slide setup which was usual used in previous versions (but that is also the case for the object styles). In previous versions the Main Master Slide objects were not set to display on the Title Master slides nor the Blank Master slide. The Title has not to be included in a theme, but the Blank master slide is compulsory and I recommend not to take over the Main master slide objects because Blank is used for software sims and PPT import. For the quizzing master slides Main Master Slide objects can be included, but you have to be careful for those master slides that they do not bump inot the embedded objects on those slides.
However for Fluid Boxes, you should never put objects on the Main Master slide because it is not supporting Fluid Boxes. Only the color could be taken over for such a responsive project.
I have been blogging about themes, and Master slides with Fluic Boxes.
As a user it is not my job, but know that I started blogging 10 years ago out of frustration with the bugs and lacking topics in the Help. Nothing much has changed since.