Like people, sometimes animals just don't like each other.
Maybe some short socializing sessions will help.
Get some good food (canned, something they don't usually have) and start by feeding them on opposite sides of the same room. If she goes to attack, simply pick her up and put her back at her dish. Don't say anything. Just pick her up and put her down.
I should mention to only feed small amounts at a time, as you will be doing this a couple of times a day.
Start off with twice a day. Then, as you observe them to be relaxing in each others company, gradually increase the number of times a day you feed them together.
Also, as they are tolerating each other better, move the bowls closer..
You want to work up to them eating out of their dishes next to each other.
Eventually, you should see less aggression as the presence of the other cat is equated with food.
At the end of each session, YOU separate the cats. This way there is no attacking.
I hope you find this helpful. Patience and consistency are key. Like I said though, sometimes they just don't like each other.