Have you tried, for a few days to see how it works, NOT having the food in front of her at all times? Some cats' appetites are much more piqued by the COMING of food than by having it sitting around all the time like furniture.
And use a good brand, not something like Meow Mix that is mostly vegetable-based fillers. Look on the bags for suggested serving size per 7 lb cat -- pick the brand that has the SMALLEST serving size measured in cups, because that will (usually) be the most nutrient-dense brand and thus she will be getting maximal calories per mouthful.
Some cats are just biologically more inclined to skinniness than others, though. I've owned two of those over the years (one did not stop looking like an alley cat until he was probably 6 or 7), and they're frustrating but that seems to just be the way they're built/wired and perfectly healthy on it. So, do what you can, and obviously if she seems pathologically skinny or it's making her tired and droopy then it *is* a problem, but also it's worth keeping in the back of your mind that not all cats naturally 'want' to be normal housecat shaped, you know?
Good luck,
Pat
