I GOT IT
You haven't made the changes to the breakpoints below the highest one
Materialize is a "Mobile first" framework which means you start at the lowest breakpoint to make your changes and work up to the highest one
If you work the opposite way round then you need go down to the next breakpoint down and make your changes repeat until you get to the lowest breakpoint. The problem doing it that way it is work intensive and it might not save the changes to the highest breakpoing, so you need check.
Much easier and quicker to work from lowest to highest breakpoints, ALL THE TIME,
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