Random Minecraft Seed not containing new biomes

I often search for nice seeds on the internets but then I think I want a unique seed for my server and not one that is already in use by many other servers.

When I generate a new map with a random seed (leaving the field blank), the server builds a map prior to the version (I think it's 1.7) which does not contain MESA biome and generally the stuff added since 1.7.

My expectation is that a map created by the newest version of the server also contains the new stuff. Now I'm running the most recent server versions but still have old maps, although I just created them.

I have two questions:

• Is it just bad luck?
• Is there a way I can integrate the missing biomes randomly into my current map without losing chunks we already built on?

I'm using the most recent spigot with only a little amount of plugins. When I put in a seed found on the internet, containing the new biomes, spigot / the server does not complain and happily generates the map with the new biomes.

Edit:

Thanks for your answer. The suggested tool showed the missing biomes - I just didn't look far enough.