Are you using loose sand in your bag?
I've set my bag up with loosly filled heavy duty bin bags double wrapped in duck tape at varying weights, 5 at 1kg, a 5kg, a few 10kg's, a 20 a couple of 25's. The sand still shifts beacuse they're loosely packed but makes changing weight a doddle. I am managing to trash the kit bags I bought off amazon regularly though, might have to invest in a brute force bag at some point.
If you're finding SS isn't working for you it might be worth checking out Coach's stuff. There's loads of workouts on his website, and his books are great.
Prehistoric, meaning shit at hunting.