This is pure speculation but I think that biomes need to be big enough to spawn certain things, so if they're very thin or "peppered" around another biome they won't spawn those things because they don't have enough space. I say it because I was in a map where I had a huge meadow continent with very small patches of black forest which didn't spawn trolls or dungeons and even copper was very rare. Tin on the other hand, was plentiful because just a small patch near a river and I had like 3 or 4 veins.
Same happened with the swamps. I had to sail far to find the first one, but it was too small. Until I walked deep enough for dungeons to start spawning and managed to find 3 in the same big area.

Now I changed maps and I have big biome areas and everything is plentiful. I've even been able to build several smelters because now I'm not sort on cores.