Does the problem occur when you begin an edit? If so,
Check if you have overridden the DataGridViewTextBoxCell's InitializeEditingControl Method. You must initialize the control's value inside it.
Here's a typical implementation
public override void InitializeEditingControl(int rowIndex, object
initialFormattedValue, DataGridViewCellStyle dataGridViewCellStyle)
{
base.InitializeEditingControl(rowIndex, initialFormattedValue,
dataGridViewCellStyle);
TextBox ctl =
DataGridView.EditingControl as TextBox;
ctl.Text = (string)this.Value;
}
If this is not the issue, share your code. I might be able to help.