I may be entering the discussion a bit late but this could show a low battery condition - since most of the photos needed flash then flash recharge time is needed between photos. This is the only instance I know in which the camera wouldn't trigger.
Next time set the ISO to a higher value and avoid zoom and flash. Make sure that AF-assist is activated in the menu or try manual focus (with CHDK this is easier, you get to use the zoom-rocker).
