Fancy feast cans are pretty small. At least 6 of those a day is what my indoor/outdoor cat eats, plus a bowl of dry. My indoor girls eat 4 cans between them per day, plus a bowl of dry but he's a lot more active than them and prefers to spend most of his time outside, even in this cold. Animals out in cold need more fuel. I have a couple of strays around now too that are always looking for chow, so I'm forever filling up the outdoor dishes.
I would try to find another variety that she likes too, I don't think an all fish diet is that good for a cat, but I know how finicky they can be for sure. Try supplementing with some dry food, mine like orijen, but not a steady diet of it, so I mix it with fancy feast dry.
I buy 3 - 4 cases of 24 cans of fancy feast a week for my crew, but they won't touch any other wet food and the fancy feast isn't a bad food IMO.
clm