It's nice for a boss to also be a "friend" but it tends to get messy in most situations.
Miyah wrote:
Among my co-workers is my boss who seems to be very close with a few of my co-workers...
[emphasis added]
Your boss is only very close to A FEW of your co-workers. Hence, her not reciprocating to your offers is not singling you out.
Yes, she probably does not feel a closeness to you to count you as one of her inner circle.
Since accepting your offers would imply (out of good manners) that she reciprocate, she SHOULD refuse you if she does not want to incur the social obligation to return the offer in kind.
Now, if she was connected to everyone BUT you, I'd see a reason to be concerned.
Just keep your relationship amicable and professional.