StringGrid Problem.

I have some problem solving a strange Grid behavior :

When i dbl-click on a cell, i open a dialog to enter some info.
When i close the dialog the focus is returning to the grid but
i can not enter any info into the cell, also when i use the arrows
to jump to another cell i still can not type anything.
Only when i click with the mouse in the cell , i'm able to
enter info.
I have tried many things to solve this but nothing helps.

Any suggestions ??