I believe I had the same problem, the problem lies within the .txd not the .col or the .dff!
TXDWorkshop is sometimes unreliable and G-TXD can fix this problem, what I done was exported all of the textures from the TXD in TXDworkshop and then created a new .txd with G-TXD , imported all of the textures and saved, trust me give it a shot.
P.S. if you have a 64 bit OS the G-TXD application will not work unless you run command prompt as an administrator and paste this code! Good luck.
| %windir%\Microsoft.NET\Framework64\v2.0.50727\ldr64.exe setwow |