The preview of the selected slide and the whole scene or project varies!

It sounds like you're using the same variable to control different slides. Sure, it works when you preview one slide, because that's the only slide using the variable's value. But when you adjust the value of a variable, it retains that value as you go through the course—until another trigger readjusts it. 

So, for example: assume you have a text variable that starts blank. On a slide, you change its value to "right." The value will still be "right" when you advance to the next slide. 

Thus, since you need to control separate interactions, the fix is to use separate variables.