This is a known issue, with a simple (if slightly tedious) fix.
When you introduce a new phenotype, such as flying, all existing clothing/armour parts work with it automatically, except robes.
Every robe model must be explicitly duplicated for the new phenotype.
Say, for example, you're using a robe model named 007. You will have a series of model files for the normal phenotype named p**1_robe007.mdl, where ** is the gender and race (pfa1_robe007.mdl, and so on).
If, say, the flying phenotype is 16, you have to copy all those models to a new series names p**16_robe007.mdl, changing all internal references to the model name. Repeat for every other robe.
Fortunately, there are tools which let you do the entire job in one batch. Copy all the original robe models into a folder. Decompile (if necessary) using nwnmdlcomp with wildcards. Then use NWN model renamer to rename all the models and fix all the internal references in one pass.
If you are doing this for CEP or whatever, you may find that some of the robe models already exist for phenotype 16, so you just have to do the missing ones.
Caveat : there are a few robe models with complicated node mappings that don't look good if you simply copy them. According to my notes, in 1.69 and CEP these are the long coats and cloak robes (robe models 24, 25, 116, 174). My solution is to use those robes only for those NPCs who never need special phenotypes. A really good modeller could provide a better solution, but since there are so few of them, it's not a showstopper.
