There may be several reasons she sleeps better in the bouncy...
As one person suggested, they like the "tight" feel of the bouncy, where it surrounds them more. Have you tried to get her in a good, tight swaddle? That can be amazing to help some kids sleep! My daughter has had some sleep issues with her youngest, but her hubby (he usually works out of town 5 days a week) can manage to get her in a good tight swaddle.... she slept ALL night for the last two nights this way! (She is 8 weeks old now....) His company has the crews working local for a while, now, so he is home every night.
Also, she may sleep better in a slightly upright position... some babies, especially those with reflux, sleep much better if they are a bit more upright.