VCO can not start up when it is put into the PLL loop?

If I simulate the VCO alone it starts oscillation very easily using power down control.

But if I put this VCO into the PLL loop it can not start up. What could be the reason?I tried to adjust the convergence parameter but no help. I have a power down control.

Thanks.

Is the control voltage proper? It may be that the variable capacitance diode is forward biased.

Thanks, Flatulent

The tank cap array is MOS varactor and it has always capacitance even negatively biased. I will try to give the tuning voltage a initial value.