Why does it never rain on my Minecraft server?

If an administrator previously used /weather clear 99999 it's possible that the timer is still going. You could reset it by setting the weather to rain for a few seconds and it should go back to normal.

There are also some biomes where it doesn't rain - all the dry biomes, even in vanilla minecraft.

I always forget Savanna counts as dry when it has grass and trees everywhere, if your base is in such a place you'd never know when it was raining elsewhere.