Cell Item Template not working after a certain number of cells.

Hello,
I've downloaded a trial version of you Silverlight Grid and I'm currently evaluating the purchase by adapting it to a real-world customer request.
The grid I'm building implements, in some cells a specific CellItemTemplateKey. To set it i'm using the following event :

private void OnQueryCellInfo(object sender,GridQueryCellInfoEventArgs e) {

[..some logic...]

e.Style.CellType = "DataBoundTemplate";
e.Style.CellItemTemplateKey = "MyTemplateKey;

}

Now all of this works fine, until the number of rows raises. After a certain number of rows are added to the grid, all templates disappear and the cell content goes back to standard cell.

Any idea what is going wrong?
Thanks
Gianandrea