There is information on what to do to fool the Sim into accepting a skin it decides it doesn't like in the ReadMe file on my skin Mods and in the ReadMe files of Rob Wiggins User Skin Mod.
Here is what I have in mine.
Troubleshooting
All these skins have been tested “in Sim,” to make sure their labeling is correct so that they are recognized by the Sim. and appear at the proper time.
Sometimes the skin will not show the first time you try to load it. Usually, it will appear the next time you go on a mission with this pilot. But occasionally it takes three times. It seems WOFF can be a bit finicky about displaying new skins (even some of the original WOFF skins as well) and needs some coaxing. If, after three tries, it does not appear, here are some tips.
1. First, check to make sure the aircraft and the pilot’s name are correctly coded. Check the aircraft label against ones of the same type and compare the pilot’s name, spelling etc. to the name used in Campaign, EXACTLY.
You will find that, with few exceptions, the last name will be separated from the first, middle and nickname by an underline _. If the name is a title with, let’s say von, it will be Albrecht von_Somebody. There are some exceptions to this as well. Hyphenated names are done this way Hans von_Hun-Kraut.
The Campaign seems to dislike the hyphenated names like Beaulieu-Marconnay and Gluszewski-Kwilicki in particular.
The biggest ringer in this name game that I have encountered was Viktor von_Pressentin von Rautter. The two vons confused me and the name shown in Campaign does not give you a clue as to the placement of the underline_.
If the pilot has a nickname, it should be within single quote marks, like this, ‘Snotty’ or ‘Stinky.’
Note: You must check these skins in the Campaign mode not in QC. A skin will appear in QC as long as the first part of the coding is correct. Example, off_Fokker_DVII_ace_t_Jasta 4 1918_, regardless of whether or not the pilot’s name is correct according to the WOFF label for him.
2. If you’re sure the coding is correct and it still doesn’t work, exit the Sim. back to WOFF shell, rename the skin in the skins folder, go back into WOFF shell, re-enter the Campaign, select a mission which includes this pilot and then launch.
3. Still no dice? After you finish cursing, rename the pilot with the name of a pilot from the “other side” of the conflict.
Then start this skin in QC, exit back to the shell, rename the skin to your regular pilot’s name and try running it again in Campaign.
4. If 3 above does not work, you can also change not just the name, but the whole skin label to an enemy plane such as a Nieuport 17 or spad13. Create a pilot in an enemy unit and run a mission with this pilot.
Now…rename one of the skins in that squadron or Escadrill slightly by adding an e on the end of the name or something. This will be your test name to try to get the Sim to recognize your skin.
Completely relabel your skin with the name of this enemy pilot and start a mission with your enemy squadron.
You should get a very strange looking bird, a Nieuport or Spad with a crazy quilt skin from the German aircraft.
I usually just go straight to this option rather than trying item 3 above.
Here is a screenshot of what you should see.