I noticed that it didn't rain in some darklands variant biomes while it rains in other, I don't know if it's normal.

The Darklands Plains and Mountains doesn't have any rain in them. I might switch the mountain to snow, since they're mountains, but to be in par with vanilla, no rain in the plains.

There is rain in plains, it's only in desert and savanna biomes there is no rain.

Yeah, just realized that. Not sure why I disabled rain in the plains, but there was probably some reason for it back when the additional Darklands biomes were implemented.

So will you add rain to darklands which haven't rain ?

Most likely.