I just recently had this problem, it was due to the texture being imbedded into the model itself which by-passed the .2da array. So if you hex-edit the model to replace the old texture, with the new one it will work. Be sure to make sure your new texture is the same number of charachters as the old texture otherwise I've found the model will cease to work. You may find this Hex Editor (
http://mh-nexus.de/en/hxd/) useful, you can find and replace all entries and then save, it will also automatically make a backup file of the pre-edited file. Hope that helps!