I know it's been a while since you solved this issue, but we're facing the same one and have no clue on how to solve it. From my understanding, the activate sheet trigger only happens when the sheet is activated the first time, but once in the sheet, activating other objects doesn't trigger anything.
How do you manage to trigger every time a tab is selected? If you could provide an example it'd be very helpful. We're using QV11.
Either I'm going nuts or there's no such option on QV11. I guess that those Object Event Triggers you're talking about were removed in version 11, as I read on this post http://community.qlik.com/thread/39913