Let's back up. Are you spraying Vetericyn, or do you have the eye gel? Is she washing her head frequently?
I would consider making sure she has a pot of water with 1 tablespoon of apple cider vinegar per gallon for head washing. ACV is said to have mild antibacterial effect.
You may need a buddy to gently hold her while you take a close look. You might use a mild saline solution to rinse the eye then immediately look for any signs of a particle in the eye that could have started this problem.
Some ducks just don't cooperate, I know that. But if you can get her to keep rinsing her own head, she will do some of the work for you, and if you can get one good look at that eye, you will have a better idea on what is next.