ModalPopupExtender and form focus problem

I have a textbox and a gridview wrapped up in an asp:UpdatePanel for a ModalPopupExtender that is being activated server-side.

I scan in a barcode and the value is loaded into the gridview.

The problem I am having is after the second barcode is scanned (and another row is added to the gridview) the focus of the webform seems to kind of "disappear". I have to click on the tab button for the text box to gain focus again. I've checked my tabindex and it is correct. I've tried forcing focus but that doesn't work.

When I remove the asp:UpdatePanel and ModalPopupExtender controls the scan, read and loading of the gridview works perfectly.

Any ideas on what could be going on?

ASP.net 3.5

thanks!