System > Multitasking > Snap windows
Turn it On. If it is already on, turn it off, then on.
If that does not help, try rebooting windows.
(You can also find "Snap window settings" from the Search bar in the task bar.)
When I installed a Samsung Smart monitor, my Windows+Left started to malfunction occasionally: I was able to move a window from right to center but not to the left end, even though "Snap windows" was on. After turning it off and on again everything worked - until the next time. No rebooting is needed, fortunately.