"Random" Linker Errors?

I don’t understand this…

I have written some code dealing with SkeletalMeshes, SkeletalMeshRessources and what’s connected to them. It’s all fine, but when I try to call any initResources()-functions on a SkeletalMeshResource, I get a linker error. Other members and functions work fine.

I had the same problem when trying to “unpack” FPackedNormals (TangentsXYZ inside Vertex-Class). In order to avoid getting linker errors here, I had to look up function definitions, copy some code into my source files and replace all references to a constant (GVectorPackingConstants) inside RenderUtils.cpp with the value of the constant.

Can someone tell me why this is happening? Can I do something about it?