What types of stretches are you doing? Make sure you hold them for a sufficient amount of time; when I first started cheering my coach would only have us hold stretches for ten seconds and it really didn't do much. Hold them for at the very minimum 30 seconds and even then that's not much. Don't cheat either. Even if it hurts, which it will, keep practicing and it will get easier. Also, know what your breaking point is - don't force yourself into any stretch if you truly can't do it because you'll end up tearing something.
When I was working toward my heel stretch, I would put my foot on one rung of the ladder on my brothers' bunk beds, hold my foot with my hand, and hold that for 2ish minutes. After a few days of stretching I would move it one or two rungs higher and do the same thing, and keep moving it higher until it was straight up. I'd also do partner stretches, putting my foot on someone's shoulder or having someone push my leg toward my head. Helped a lot.
I've always been told to sit in your splits and grab your foot; maybe I was doing it wrong but that never helped me... it might help you though, so it's worth a shot.