Possibilitties:
- texture has different name or is inserted in different path than model is looking for (if you have hardcoded texture)
- texture has wrong name, or there is a wrong name is in itemdisplayinfo.dbc (if you don't have hardcoded texture)
- texture isn't in item/objectcomponents/weapon/ (if you don't have hardcoded texture)
- texture doesn't have permitted size (both height and width have to be one of following: 4, 8, 16, 32, 64, 96, 128, 256, 512, 768, 1024, 2048... and maybe a few other combinations (like 32+64=96, its not from 2^x but still works).