I also have a chicken like this (year and a half old, pecking order is fully established). When I bring the treat dish out, she kinda waits for the others to finish their frenzy and then maybe she'll pick at it. But she's also fine just going back to the pellet feed or the nest box while the craziness is happening. I have tried multiple feeders, she will come around but in general she'd rather stay out of the fray. BUT she does have a favorite treat she will go nuts for - "Golden Nuggets". I dunno why but she likes them more than anything else I've tried to give her. And I have another chicken that has no interest in them. They have their preferences!
This is the Golden Nuggets, in case you were wondering -
https://www.chewy.com/farmers-helper-golden-egg-nugget/dp/248665
She was always the shyest out of the 10 in her batch of babies. But she's very sweet and seems to be doing fine, even if she is alone most of the time. She roosts with everyone and hangs around them, she's just not buddy buddy and doesn't want to get involved with all the hub bub. I don't blame her. (Also- she is the biggest and fluffiest chicken I have, and she's no where near the top of the hierarchy).