We've figured out this "emitters not doing anything" problem.
For some reason, some super emitters don't create free emitters (and therefore don't create any particles) at different framerates (I usually design emitters at 30 fps).
So if your project framerate is set to 24 fps (for example), those certain emitters won't show. You have 2 solutions:
1) Change your project framerate to something else that works (not a great solution)
2) For emitters that don't work at the framerate you want to use, just increase the topmost "life" value a little until you see particles. (Add the emitter to the stage, move ahead a few frames so there will be something to see, select the topmost "life" property in the hierarchy, then click and drag the data point in the graph window up a little until the particles show.)