Go out early while it's still dark and she's roosting. Take a towel with you. Grab her off the roost and lightly swaddle her in the towel capturing her wings to her body. This makes it easier to control her.
Can you gather her up and inspect the eye closely to see if she had debris, pus or injury to the eye?
It may need to be cleaned - flush with saline.
Possibly she's gotten dirt in it or she was pecked.
While you have her up, give her a going over for lice/mites, take note of any symptoms of...