taskbar > show windows side by side not working

[jeffk]

Windows 10 Home 64x

When I click on the taskbar & select 'show windows side by side' it shows them stacked instead. It doesn't work the other way round - when I select 'show windows stacked' it shows them stacked, as it's meant to.

Any ideas why this is happening & how I can get it to work as its meant to?

Thanks