mirroring features in a weldment

hello - a newbie here so if this has been discussed beforeplease direct me to the answer if possible - i am creating aweldment of a number of plates & blocks - i wanted to mirror 1plate & 1 block about a centerline to have these featuressymetrical on both sides - i was able to mirror the features butbefore finishing the command it told me that they would have to bebodies or faces or to un-check the geometric pattern - i did this& it would still not complete the command - how do you mirrorfeatures in a weldment that were created from sketches on otherfeatures?? - i am using sw09 - thank youSolidworksParts And Features