Sometimes with activities you will need to really engage those muscles. Think about squeezing your booty as you do the squats. Maybe try a few other work outs that will further target your butt better.
I can't remember what it's called for the life of me, but kneel on all fours. Extend leg to the back and straighten. Swing your leg out to the side while maintaining the 90 degree angle to your body. When extended to the side, pulse up towards the ceiling. Then bring leg back behind you. Rep 20 times on each side.
You can do this exercise standing and resting your hands on a chair. I choose kneeling because it doesn't strain my lower back as much.
It hurts like a mo fo but it gets the job done.
I have the same issues with ab work outs and can't rely on my back to do all the work. Which, by the way, is by wrong.