First, why are you filling the DataTable when the user selects a new item in the combobox? By that point, it's too late since the user will only be selecting what is currently in the ComboBox's DataSource.

I think the problem that you're having is that you don't realize that each Form is a different class, so unless you made the DataTable / TableAdapter as global objects (like by making a separate module that declares those objects) or declare the DataTable for one Form and then refer to the Form's DataTable from the second Form, then as jmcilhinney mentioned, you are actually referring to 2 separate objects in your program and therefore have to refresh each separately. I think that you should just declare the DataTable in Form1 and then refer to that from Form2.