Clicking on an active tab causes the focus to shift to the neighboring tab. Clicking on the same tab again brings the focus back. Clicking on an active tab should never cause it to lose focus. The difference in appearance between the active tab and other tabs is fairly small, so it is easy to get confused about which tab is active.
Thanks for the suggestion. We had originally modelled it on how the Windows Start Bar works but I think we will look to make it work more like tabbed browsing on Internet Explorer, Chrome et al.