[Flash8] Filters disappear

Hi

Im setting filters to a big mc like

mc.filter = new flash.filters.BlurFilter(5, 5, 3);
mc.filters = [mc.filter];

but as soon as I change the content(loop that duplicates some of the mc’s content)
the filter disappears and I cannot add it later also, everything traces fine but its just not visible :*(

can’t I add filters to movieclips that change content?
plz help!