Contrails Enabled in Aircraft.cfg but they don't appear

Hi Folks, We have contrails enabled in aircraft.cfg;

contrail=fx_contrail_l, 0

but none appear - even though AI traffic around the plane are showing trails.
General troubleshooting steps or suggestions where to look next please?

Legacy particle effects are deprecated, you need to create them via the new
vfx system and they get installed via model behaviours. Check thr SDK
documentation, also check the default airbus from Asobo to see how it works.
Best, Raul

Thank you Raul @Simbol