What is the Frequency of oscillation.
Are you aware that you need to add an initial condition ic=(some current) in the Lm (inductor) model of the crystal, else your crystal will not start.

If it is MHz frequency try making ic=5uA. If it is kHz frequency make ic=500nA and give it a try.

Regards

ic_engr

The LRC model of the crystal in simulation is only ideal. There are no inherent noise in the crystal model in simulation to start and let the oscillations grow.

Therefore, the simulation needs an initial condition of some current in Lm.

There are other ways to start, but usually the simplistic approach is the use of initial condition.

Does your LDO have a decoupling capacitor that is large. It could be the ramp up is slightly longer such that the transient on LDO is unable to start the oscillation, contrary to the ideal supply.

Your other test is: use ideal supply as DC (don't ramp-up set DC voltage only by using VDC) then use initial condition current ic=xxx check what is the minimum you need to let the oscillations grow in simulation. May be 1uA or 5uA I don't know.
Remember if you give too large, then the amplitudes will clip therefore give lower values as explained before. If you can succeed in starting with some initial condition for a true ideal DC supply (with no ramp), then the same condition should be used for the case of LDO and your crystal oscillator should start with no problems.

ic_engr

Thank you very much for all the inputs!