Terrain detail mesh not rendering

I'm making some basic lowpoly models in Blender and importing them to Unity, but the detail painter in the terrain system does not paint these models no matter what I do... I replaced the model I made with a prefab from the standard Unity cube gameobject, and that one paints flawlessly... I changed the settings in the prefab with my own mesh to be as similar as possible to the cube prefab, with the mesh eventually being the only difference between the two. Still doesn't paint it. Why does this system not process the meshes I've made? I know the terrain system is still in development, but I've seen this work for other people, I just can't find any info on what I have to consider when making these meshes ffs...