I had similar problems naming my Variable Upright+Italic fonts. Take a look at this post you can see (screenshots) the way I set my Variable Font Setting – it works:
https://forum.glyphsapp.com/t/naming-tutorial-variable-section/24095
In the end i just changed Name: Default to Name: Regular, for the regular style. If you have different names under fileName there should be no conflict in Font Book, maybe you need to clear your font cache.