hayley i don't understand your point, if they are drawn at random but do not include a full cross section of all various age, ethnic backgrounds etc then this lack of some groups must thus represent society. For example in my area there not as many asian people as white people - so the odds of having an asian man on a jury is lower than having a white man on the jury... i think this is representative of society in real sense.
the argument that could be made for some lack of representation from various sections of society is that certain groups are not registered to vote and are thus not on the electoral roles meaning that they will never be selected for jury duty - if there was a way to discover if the people that didn't register share some characteristics (maybe income, location, socio-economic group etc etc) then you may be able to identify a group in society not represented