Ok, so you increased her grade. Higher grade slaves require more luxury, which can be gained in a few ways.
1) there are items you can buy to equip to her that can increase luxury. The first, and easiest to get, is the silk underwear/boxers from the market in Wimborn. They increase luxury for the slave it's equipped to.
2) in Frostford you can buy a Gold Ring. Identical to the underwear, but uses a different equip slot.
3) There is an upgrade to increase the luxury of all residents of the mansion in the upgrades menu. There are 2 levels to it, and it's a bit expensive.
4) And finally, giving them their own Personal Room increases luxury.
I might be missing something, but I can't think of anything else. I generally give my girls a ring and new underwear, upgrade the mansion, and leave it at that. If they still complain, I'll give them a personal room.
There are also traits that reduce, or, I believe, outright remove the luxury requirement even if they're high grade. Maple, a unique fairy slave, has one that decreases her luxury requirement, so, even though she's Noble rank, the highest one, she barely needs any luxury.
As for a list of the Ranks, if you click the magnifying glass next to a slave's name, and hover your mouse over their current rank, it'll show the list of ranks at the top of the pop-out. And I'll post the list here as well:
Slave>Poor>Commoner>Rich>Noble
A note I feel you might want to know: The higher the girl's rank, the higher her max Beauty (and other stats) is. So, when you're out hunting slaves, the higher their beauty, the higher their rank might potentially be. Also, there is no downside to dropping the rank of a slave. If you get a slave to Noble, max it's stats (courage, confidence, wit and charm) at 100, you can then drop it to slave to reduce it's luxury requirements and it will keep it's stats. But if they ever drop, you will need to up their rank to get it back up.
Hope this helps.
*realizes how long his post is and starts freaking out* I always do this! And this isn't helping! *runs off*