I created 16 decals, all using the same method. One of the decals will not appear on the model. I get a blank placeholder as though it is there, but no image on it. All the others worked fine.
Have you tried putting one of the other decals in this part?
If other decals are not showing up either on this part, try checking the option 'Use mapping shape' this using a different method for applying the decal that should work on most data.
If the other decals appear correctly on this part then I would contact tech support and log a case to investigate why this one decal is not appearing.