I have no idea if this applies to your difficulty or not but I just came across this piece of advice in a Blender tutorial regarding importing and exporting files:
Quote: "Packing Data
If you plan to open this file on other computers, you will need to select the “Pack Data” option in the file menu. Textures and sounds are not automatically included in your Blender file in order to
keep the file size down.
Every time your file opens, it looks for the textures and
sounds and places them into your model. If it can’t find the files, you won’t have any textures and sounds. If you pack data,
those files are included with the .blend file so they can be opened anywhere,however, your file size may explode.
When data is packed, a small package shows up on the top of your screen letting you know that the file is packed. You can also
unpack data to bring the file size back down."
I DO NOT know if this Pack Data option is supposed to be used when converting Blender files to .x or .3ds before exporting them, but I thought it might be a good idea to mention it.
Eternal student in search of knowledge. But will settle for the occasional epiphany.