The xenviro bug is well known. You don't get any rain effects unless you start switching the weather mode around, then eventually it starts working to some degree, but it's very patchy.
It's a real pity, coming from FSX, AS spoiled us when you compare to default, xenviro or SMP. I have swapped between these programs many times, trying to decide.
SMP looks nice and has the rain effect, but I feel like I am flying in a mini weather zone, about 10 miles wide, whatever the cloud cover is, be it 2/8 cover, you constantly have clouds in front of you and it's very predictable, instead of random placement clouds
xenviro is better in this case, but you miss the effect, the haze seems too strong.
I don't know if xenviro plan to resolve the issue. Roll on AS release!