As I reported on http://www.fusioncharts.com/forum/Topic17203-31-1.aspx, I'm thinking this has to do with the fact that most-likely one of the charts you're trying to export has not yet been scrolled into the viewport, and therefore cannot be accessed thru javascript.
Try scrolling down the page, and after that try exporting them, in my case that will do the trick.
But I'm also looking for a solution for that one...
I'm thinking about using javascript to forcefully scroll to the position in the page where the chart resides, so that it is triggered and rendered properly, before triggering the batch-export.
Not a very neat solution, but the only one I can come up with in this scenario....
Unless support comes up with a better idea