I've never actually owned a buckskin, but yes, your mare can shine, even at the lighter color. I've seen grays work up a sheen.
Just on the side, have you ever thought about color enhancement, like Gold as Sun (something like that?), SmartDark, or even straight paprika? It's all known to bring back out the golden and dapples back out. I know paprika helps my bay. Plus, those supplements have other ingredients for additional shine and overall coat health. I'm not sure about her allergies with those though....
This particular supplement might not be the one for her. Some things just don't work on all horses. I think you should have noticed something, by three months.
I would personally try a different supplement. I think it's nutritional.
"You're just as sane as I am."~Luna Lovegood.