Silversands and Jackpotcash not paying

Both Silversands and Jackpotcash casinos give you depositers vouchers that gives you a % on your deposit amount to play and only then you win big. As soon as you win they deduct wagering amount and you are left with very little. Friends have "Won" like R22000 and are only able to cash out R800. If you dont use coupons, you might as well toss your money in a trash can