If your silicone caulking is cracking it can be from no other reason than movement between the two surfaces being caulked. You will need to stop the movement as Carrust said . It may seem like a lot of effort,but if you do it your seal should last for years just as mine have. If the tub apron is properly supported filling the tub or leaving it empty will make no difference. As others have told you, a clean ,dry surface is always needed, but it won't help if there is still movement.