Precipitation can be attached to any cloud keyframe and it will produce precipitation based on cloud position and thickness (if desired). Snow or rain can be created with multiple customisable parameters.
Check you are editing the correct keyframe on the currently active sequence. In order see your edits as you procede, ensure the time is correctly set in the timeline and that the simulation is running.
Ensure there are enough cloud above the viewer to produce precipitation. If there’s no clouds, no precipitation can occur.
Double check you have trueSKY Translucent as a Post Process. (Unreal only)