Unity not finding shared materials from imported Blender models

I have several models created in Blender which I would like to use the same material. I am using the Recursive-up import setting with a "Materials" folder a few directories up and then just mapping the same image onto the various models so that it has the same name. This has worked fine for several of the models, but some will create a new material with the prefix "None___" instead of finding the correct materials.

For example, my image and material is called "GenericSkin" and some of the models generate a material called "None___GenericSkin". I can't figure out why some of the models are working and others are not, because the process I used to create all the models was more or less the same.

Thanks.

Edit: The models have more than one material per mesh, and it seems that this only happens when materials are in a specific order. If I change to one material per mesh, the naming is correct. This seems very strange to me...