Birt report on WI complexity changes in RTC 2.0

I'm trying to author a report which should show me complexity changes for stories. I would like to report that a story complexity has e.g. changed from 13 story points to 8 story points and when that has happened. I would like to report this for all stories in a given project area.

I've started querying the WORKITEMS_SNAPSHOT fact table WC_STRING_EXT using '{Complexity Attribute}' as parameter for the column KEY.

This lists some complexity changes for some stories, but not all of them.

Any hint what I need to do differently?

Thanks,

Martin