My problem is that when I go to a mountain and switch the weather to thunder, I never really get any lightning.
Other than needing a biome where it rains (not snows), lighting strikes are completely random. Every tick there is a 1 in 100 thousand chance that lightning will strike in a particular chunk. If this check passes, it picks a random x, z, and selects the highest block on that coordinate. If that block can see the sky, and rain is falling on it, lightning strikes. There is no way to increase the chance of lightning strike. (Source)