Although I'm not sure that this is the cause of David's problem, what is described would be consistent with having the 'Toggle Breakpoints' menu set to 'Disable All Breakpoints'.

With that setting selected, whatever one changes affects* the lowest viewport, regardless of where the viewport slider is. Settings specific to a higher viewport should be made with the 'Toggle Breakpoints' menu set at either 'Default Mode' or with a particular breakpoint selected.

Frank

*in 'mobile first' frameworks.

Since David was mentioning 'none', 'block' etc, I didn't think of the 'disable all breakpoint' option, but of course, If he actually had them disabled, then what you are saying might be right.