The tooltiptext property is evaluated only once when the object is created. In your case, the value is an empty string because the grid doesn't contain data at that moment.

To make it work, put the following in the MouseMove event of the object inside the grid:
This.ToolTipText = ctod(This.Value)

Obviously, you have to replace the ctod with your own expression.

But tooltips don't work properly for objects inside a grid. Probably you'll have to use something like my FancyToolTips class in order to have them properly displayed. Anyway, you'll see the tooltip only for the current line inside the grid.

Vlad