Articulate 360 Not Maximizing Properly with Dual Screens

I had this same problem.  It turned out to not be a monitor issue, or a Storyline 360 issue.  It was a display settings issue, where the scaling size was different between the two monitors.

HOW TO FIX:
Using Windows, I right clicked the desktop and clicked "Display Settings."

From there, if you look at Monitor #1, under "Scale and Layout," look for the drop down menu that is under "Change the size of text, apps, and other items."  It is probably something like 100% or 125%.

On the second monitor (which if you're using a laptop, like me, for your first monitor), you probably have the second monitor set to display slightly larger.  Mine was at 150%.

This resulted in an overlap of the windows when maximizing Storyline 360.  It also resulted in Storyline 360 cutting off the bottom part of the app window (where you can scroll right/left on the timeline).  

Once I set the scaling to 125% (as my first monitor had), instantly the problems went away, and the Storyline App is displaying properly.

Hopefully, this helps!