When trying to select a SmartMark in the SmartMark palette (eg: marks added under 'General': cell numbering, Gallus...), the selection gets deselected automatically and it is not possible to keep the SmartMark selected.
Certain SmartMarks will not stay selected in the SmartMark palette.
This behavior is because one is selecting a SmartMark object while the transform tool is active (or while the transform dialog in Plato is open). The selected SmartMark object cannot be transformed and for this reason it gets deselected automatically by the application.
This applies to the SmartMarks which don't have a specific anchor-point (the ones that appear in the SmartMark dialog under 'General'). These marks will generate objects at specific positions and cannot be moved using the transform tool.
To work around this issue, make sure to select the SmartMark in the artwork using the Selection tool. Once the mark is selected in this manner, it can be manipulated as usual using the SmartMark palette.