I've noticed that having my litterboxes in a place where the air circulates makes a HUGE difference! I live in a townhouse, and I have 1 large litterbox outside my room at the top of the stairs & 1 extra large walk in litterbox at the bottom of the stairs right by the front door. As long as I scoop every couple of days and there is some kind of air circulation (fans, a/c, windows, etc) then there is no smell. I do realize some people are have limited space though.
Also, plastic will absorb odors, so maybe it is just time to replace their litterbox?