Is this a VFX spawner from niagra or just in game? Also, is it set on looping? If not it might be finishing the VFX spawn cycle before you are able to view it. As well as it might be ENABLED, but not ACTIVATED. Are you using a trigger or event to start the VFX, or is it supposed to be a looping effect that lasts the whole game?
vfx spawner device set to continuous. I enable it whenever I want it to play (it starts out as disabled using the setting enabled on phase: None) then disable to stop.