one of two things, probably:
either the time-step is too high and objects are traveling through the sun without hitting it, or
as the objects get closer to the sun, they lose potential energy and gain kinetic energy (speed). because they were revolving, they don't head straight towards the sun, but slightly around it. because the potential energy has all been converted to kinetic energy and the planet is moving fast now, it can skip past the sun before being pulled in.