this seems to be one of the standard issue when beginning with SE
There are 2 settings You have to take care
One is that the "Keep Relations" button must be ON
This is the marked button in the top line of the tab
The other is, that the dimension itself must be declared as DRIVE and not driven
This is, the lock sign in the dim tab must be on.
You see, that a dimension is at DRIVE that it is shown in red where a driven dimension normally is blue.
The default values for those settings normally are as mentioned but somehow You or another user might changed it.
Set it back and draw Your rectangle again, give the dimensions to it and YOu will see, everytihng works as expected