I have a world that I created - survival world, no cheats enabled etc before I signed up for Realms.  I imported this world into realms and it is my only Realm.

Playing the original solo copy on Xbox thunderstorms occur with expected normal frequency.

On the imported realm version, there has never been a single thunderstorm - 40+ hours (real time) and not a single thunderstorm.  I have tried to stay for 10 to 15 Minecraft days in a various thunderstorm eligible biomes - plains, deep ocean, mountains etc and nothing - only normal precipitation either rain or snow.

I have also tried for hours in each difficulty - peaceful, easy, normal, hard - no difference - none had any thunderstorms.

Since no cheats or anything are enabled, there is not much I can do - what I have tried is copying the world back down to a solo world, set it to creative, then changed the weather to thunderstorm to confirm that every biome I tested above actually got thunder and lightning - which they all do.