Not sure does this fix it 100% but...
I had the same problem and it has something to do in the exported file format and few blender settings.
For me when exporting in .glb they came as cubes. Then i went to blender (Blender 2.8) and applied rotation scale and all transforms to the object. To access the menu in blender select the object CTRL+ A
Not sure if this fixses everything but made mine work alot better.