The exercise balls slide out because the muscles of the pelvic floor are weak. Contracting and working the pelvic floor muscles will make them stronger, and they'll keep the exercise balls in place longer.
Start slowly: You might leave the balls in only a short time, keeping your pelvic floor muscles contracted to hold them in place. (Specific how-to instructions are on our website--and we found an app that helps!) Depending on how much muscle strength you have, you may need to start while sitting or lying down rather than standing.
As with other muscles, it will take weeks to build strength in those muscles, but it definitely happens! Exercise balls or other Kegel tools help by making it easier to tell that you're flexing the right muscles. And the payoff is big--not only does a strong pelvic floor counter incontinence and hold your organs in place, it can also strengthen orgasm.