I'm not exactly sure what you mean by "simulating start up failure".  Presumably you are trying to give it a good kick to get it started and see if it can sustain the oscillation?

I have see people use three ways to start an oscillator:
1. using initial conditions
2. using a tickler source
3. using transient noise

Using transient noise is the least reliable and least efficient.  Using initial conditions tends to work very well for ring oscillators.  Using a tickler source tends to work less well because people often put the source on the supply, and the mode of oscillation is not very sensitive to perturbations on the supply.  This is particularly true for differential ring oscillators.

If the oscillation does not sustain, it still might not be an issue with the circuit.  Simulators can exhibit numerical damping.  To reduce the chance that the simulator quenches the oscillation due to numerical damping, set maximum time step to get at least 30 time points per cycle and switch the integration method to trapezoidal rule.

-Ken