You only have one chance at loot from each boss in LFR once per lockout. Whether you win anything or not makes no difference, and no other successive kills of the boss have any chance to give you loot. So in your example, going for Mogu'shan Vaults LFR a second time, there is no chance whatsoever that The Stone Guard will drop anything for you. You need to wait until the next reset to get another chance at loot from The Stone Guard.
Now, all of that does not account for the Elder Charms of Good Fortune. An Elder Charm of Good Fortune is basically a second chance at getting loot from a boss you killed. The chance, according to Blizzard, is exactly the same that you have the first time around. For example, you have a 15% chance to get an item from The Stone Guard the first time you kill them in a lockout. So it's Wednesday night, you go in and kill The Stone Guard, and nothing drops. You had (roughly) a 15% chance to get an item, but you didn't. Now you use an Elder Charm of Good Fortune, and you get another chance to win an item from them. The chance is again roughly 15%, and you could end up getting an item, or not getting one.
Finally, it is my understanding (and if someone has a blue post to contradict me, please go ahead) that the Elder Charms of Good Fortune are not subject to the limitation of one chance at loot from each boss in LFR once per lockout. That means that each time you use a Charm, you always have that 15% chance to get an item, no matter how many times in the week you've already killed that boss before. > see Peelyon's reply.
The Elder Charm of Good Fortune is usable once per lockout per boss. So, if you kill The Stone Guard (first time in the week), and use a Charm, you won't be able to use another Charm on The Stone Guard if you kill it again the same week. However, if you kill it the first time, don't use a Charm, you will still have the option to use a Charm on subsequent kills of it in the same lockout (until you actually use it).