James,
It seems you have great difficulty understanding how this works. Martin explained this to you pretty well already...?!
The default apron textures in the afcad provide the rain effect. When custom textures are placed on top of this, the rain effect no longer becomes visible. So over 2 years ago cloud9 and flytampa began making their custom textures slightly transparent, so that the ground appears all custom, but you can still see a little bit of the default apron and thus the rain effects.
Fsdt sceneries and rain effects:
ZurichX: Photoreal background provided by fs2002 ground polygons. No transparency in the textures, so no rain effects.
KORDX: Photoreal background providded by fsx-native photoscenery. There's no way to do rain effects on this type of ground (unless default aprons are used, but that would ruin performance and appearance)
KJFKX: same as Kord
GreystoneX: same type of photoscenery as Kord, kjfk, but has default aprons, so has full rain effects
LSGGX: same as Zurich, but with transparency in each custom texture, so rain effects show.
KLASX: same as LSGG
So it's not your settings, you have to take into consideration how each scenery is made. You might ask, why didn't they make KORD and JFK like LSGG? Reason: performance. There have to be some trade-offs, unfortunately.
Regards
Ron