The combined mesh is different between static-batched objects?

Hello, I'm working to optimize my mobile VR game using the Profiler and Frame Debugger. I'm seeing this message quite often in the frame debugger, and I'm not sure what it means?

Why this draw call can't be batched with the previous one

The combined mesh is different between static-batched objects.

The static meshes giving this error are simply duplicates of other meshes which do batch together properly. The other "why can't it batch" messages all make sense (I have a ton of small rocks which go past the vert/index limit, for example) but the meaning of this one is escaping me.

In one case, I have mountains which use 3 different materials for the bottom, middle, and top. For each of these materials, they are sorted and a batch is created - but actually there are 2 batches for each mat. It seems 1 or 2 mountains are rendered, followed by ~15 others on the next draw call. This second batch is giving me the above error message, as to why they weren't all batched, but I just don't understand it. Just for this example alone, that adds 3 superfluous draw calls, and it's happening on various objects throughout my scene.

Does anyone know what to do? I'll keep investigating but thanks for any help!