Without seeing what you are doing, it is very difficult to advise, but you need to aim to have your body weight right over the ball of foot when making a turn in Rumba. Try and dance through a few turning figures, and see if you can just stop moving altogether, instead of turning. Do you wobble, or are you balanced? Remember to avoid turning while stepping: in most figures, the turn is an action that occurs between steps, rather than on them. Think of a simple Rumba Spot Turn, and take the first step, across the body, gather your weight over the ball of foot, and then make a controlled half turn, using the toe of the other foot to help your balance, before the weight change, in place, and then step forward again, before turning back to your partner. In the initial forward walk, turning, and in the final one, the feel is of a forward step, followed by a swivel. It will help to develop that action (if you need help with it), to make sure that both forward steps (taken in opposite directions, of course), are both with toe turned out - that will help to stop anticipating the turn, as you step.
I may be wide of the mark, having not seen you dance, but you may get something from the above...