We've had the same problem, and we finally figured out it was a food stain...from our kid's food (esp anything made w veg or olive oil, or that has it in it). We have a 2 y.o. and a 10 month old, the stains started when our 2 y.o. started eating solids (not jar food, more like table food). The best remedy I've found is rinsing his stained shirts before putting them in the wash, removing any food residue prior to washing. Like you, we didn't have much luck with stain treatments after the fact, but if you get to it before the wash that should help. I can't say that we still don't see these mysterious stains every once in a while, despite our efforts. But it is NOT something in your washer (unless it's left over food), the washer isn't broken, and there isn't an invisible grease monkey in there! ;)
Best of luck!