Are you giving her extra protein? Meal worms, dry dog food [good quality] some use cat food. The extra protein is good for feather quality.She preens constantly! Spends way too much time at it. Mostly because of how wet she is. And she gets so little out of it. Without sun or a light (hopefully warm) wind, she just does not dry. I feel so sorry for her. It seems to take her away from eating while the others get their fill.
The issue is not that she is not motivated to try to keep herself clean (and both of my other 2 do a GREAT job), it is that she does not seem to realize she has and should use the gland. It's almost like she just does not know it's there. That's why I tried to show her. My 3 (one is now gone) other muscovy use it instinctfully. This one just hasn't a clue. She also is what you could refer to as a runt. Very small (compared to the other hen) and I think was very used to being mated by all the drakes (regardless of breed) at the farm. In other words, she was very easy pickins and when my drake jumped on her the instant they met, she was ready to submit - she clearly had it down.