Mark,
Frankly, when I go to clean a suite, I prespray the MS on the dirtiest part, agitate, then rinse with water. If Im unimpressed, I repeat, if that works I carry on. If not, I usually change to Ultimate master for the whole suite. I do very often find that the second application of MS does the trick. Why, as you say, the soil doesnt always split with the first application, I dont know, perhaps those that do can enlighten us?
Dave.