Elaborate on what you mean by they dont show up in game. Are they displaying the grey grid material, not visible at all, only visible in certain cases, etc? And what are you trying to do with the rocks, are they a placeable item or what? And lastly, when does the issue occur. If they are placeable can you see the greentrace or not?
These kinds of issues its very hard to figure out the issue especially remotely if we only have the info you have given. By composite mesh are you saying you added multiple meshes as components to one main mesh? If this is the case you shouldn’t have any visual issues if the mesh involves only static meshes and no animations are used. Make sure all of the static meshes you used are either included in the vanilla game files or your mod folder. If they are not vanilla game files you will need to include the mesh, material, and textures for each mesh used.