scenes/slides navigation

There are two reasons they can be side by side. One is that you have a main menu slide, with triggers to go to different slides, and the different slides have no way to go to each other. In other words, from the menu, you can go to a slide or slides (a branch) and come back, but can't go from branch to branch. In that case the branches will be side by side under the menu. If the branches are longer than one slide, the branch will be vertical within itself, and side by side. Here is a sample of a menu slide with three branches. The first branch has two slides, and the other two, only one each. 

The other cause of slides being side by side is if there is no trigger to jump from one to the next. If there is, the slides will be vertical, if not, they will be horizontal.  here is a picture of three random slides that have no connection to each other. There are no triggers to jump, and Next and Previous are disabled.

This is the same slide show after I enabled the default Prev and Next.

I personally never use the default Prev and Next. I have more control with my custom buttons, and my projects don't look like everybody else's cookie-cutter projects. I use a text box, or a shape (if I want rounded corners.) I can customize the colors and text far easier, I don't have the built-in baggage of the default buttons, and I have far more precision than the default Jump to Next ... .

 If "once getting the question correct" means you are using some sort of quiz, there may be another reason why it is not advancing. If you use the built-in feedback layers, they already have the triggers built into them to advance. If you choose not to have feedback, or build your own custom feedback layers or build your own custom quiz from scratch, you will have to build in the jump triggers.