I think you may want to check the following:
1) have to inject impluse current at the output node of the VCO to initiate or speed up your oscillation.
2) You may have to run a longer period to check whether your circuit take a longer time to start oscillating.
3) To limit your maxstep in the time step parameter in the option of transient analysis if your are using spectre
4) Another method is to ramp up your supply voltage in transient analysis.
One thing to point out, if your transient analysis is not oscillating, are you sure that the result you get for PSS/Pnoise is working fine. In PSS, you should be able to plot the output waveform too, can you confirm that the output is correct.