There's a few things to consider here:
1) In order to turn a flat bum into a round bum, you need to build your glute muscles, especially gluteus maximus.
2) p90x is generally targeted towards fat loss more than muscle building. To a large extent, the difference between the two is diet, rather than how you train, but it needs to be stated that the more you want to build your bum, the more you're actually going to have set your sights on BUILDING, not on fat loss.
3) Due to our lifestyles, most of us have rather inactive glutes. It takes some time and effort to learn how to actually fire up your glutes and get them working while you train. If your glutes are asleep, then even if you're doing exercises that supposedly work the glutes, other muscles will be doing the job, and your glutes won't get an overload, which they need in order to grow. So start with learning to activate your glutes, and then do glute-specific exercises.
4) In order to build a muscle, you want to be practicing progressive overload. That means that if you've been doing, say, 3x10x20lb on an exercise, then you want to be working up over the following weeks to 3x10x25lb or 3x12x20lb, etc.
5) Back to diet, people (and especially women) are always taught that if they want to get fit they need to eat less. But(t) if you want to build muscle, you'll actually want to be eating enough to gain about 1lb/month. More than that will mostly be additional fat; less than that will likely be restricting the amount of gluteal muscle you can develop.
6) Everybody's different. Consequently, once you've got your glutes activated, you're overloading them, and you're eating enough good food to enable growth, less controllable variables such as the ratio of muscle fibre types and hormone levels will have a strong part to play in how effectively your specific program works for you. You may have an easy time building glutes once you've got all the controllable variables right. You might have to work at it for a year or two before your glutes are where you would currently like them to be (at which time your goals might have changed, anyway). The opportunity to improve is definitely there -- it always is -- but it may be a longer road than you've been expecting. If you're planning to have long term results, though, that's okay, because long term results are only for those who continue training long term, long after p90x.
oh female by the way