That's good advice above about doing some MFR on the hip flexors, quads and inner thighs.
You can also start doing some hip flexor stretches before and after workouts. Do them before for 1-2 sets of 5-10 reps, each lasting a 2 count. Afterwards or at night hold the stretch for 2 sets of 20-30 seconds. Keep your abs and glutes tight while you do these.
Next, do some glute activations before your workouts. You'll want to try to activate the three glute muscles and as much of the glute max fibers as possible. These include things light
Banded duck walks
Banded lateral monster walks
Supine (single and double leg) bridge
Bird-dogs
Etc. Concentrate on getting a good squeze when you do them, and keep your abdominals tight to support a straight back.
Next, we can start incorporating more direct glute work. Squats and dead lifts are great, the but the glutes are most intensely activated when performing hip hyperextension, which you cannot accomplish with axial loading (ie. weight on your back like a squat or lunge). A couple really good moves are hip pull throughs and glute hip barbell thrusts. Both load the weight horizontally and allow for hyper extension. You can do the barbell glute bridge double or single leg, or with your back on the floor or a bench. Donkey kick backs are also a good exercise.
As I searched for pictures to illustrate this threat, the final one came from this article. I think reading this will give you some novel moves that will provide good results.
http://www.t-nation.com/free_online_article/most_recent/glutes_gone_wild
I also think you need to vary your sets and rep ranges. Body pump is great for cardio, but it is not going to provide the stimulus to build muscle mass that you need to develop a nice butt. I would work 2-3 days a week with heavier weights, in the 6-10 rep range. I list a sample work out as well as the rationale as to why women need to build muscle to develop a sexy feminine physique in the article I posted above, but here it is again:
http://jasoncholewa.com/2012/12/17/optimal-beach-performance-for-women/
Br