The Export Log will show any errors that occured during export.
Missing Texture [texture]
The texture is not where the filename says it should be.
Texture Width not Power of 2 [texture]
Texture Height not Power of 2 [texture]
The object uses an unsupported material. See Materials...
Object [obj] has invalud Sub Material [mtl]
The object uses a Multi/Sub-Object Material that contains an unsupported material.
Invalid Cubic Map Width, should equal 6 x Height [texture]
Cubic maps must have a 6:1 aspect ratio. See Cubic Maps.
Could not construct Relative Path [texture]
This generally happens when the texture is on a different drive than the b3d file.
XRef [xref] not in Data Path [dataroot]
The XRef path did not contain the data root directory. See How To Organize Projects