This is not a very concise error description.
KiCad version, ngspice version?
Eeschema/ngspice result (answer by ngspice to your simulation input)?
ngspice netlist (post here as text): Eeschema → File → Export → Netlist… → Spice → Export Netlist
Oscillators are bistable circuits. Sometimes they just do not want starting the oscillation in a simulator because the internal noise is too low, in contrast to the real world.
Sometimes it is just helpful to simulate for a longer time, oscillation will start later.
Sometimes they might need a kick to start oscillating, e.g. a small dimple in the powersupply voltage, offered by a short one-time pulse.
In Simulation examples for KiCad/Eeschema/ngspice, after scrolling down a bit, you may find some oscillators.