OP, I have a similar situation with a couple from our church. We've been attending this church for 5 years. There are several families who have children the same age. We all often find time to chit-chat during church events. Our kids all interact in church activities. All of the couples are friendly to us -- except one. DH and I haven't been able to figure out what the problem is.
The woman, Erica, has acknowledged DH's presence in the past, but she does not ever acknowledge mine. And by acknowledging DH's presence, I mean saying, "Hello." That's about it. I should note that she no longer acknowledges him. When I have smiled or said hello, she will look down her nose at me as if she is amused by my presence. Then she will immediately look away or walk away as if I said nothing. Clearly, she is sending a message. The message I get is that she perceives I am beneath her. I have no idea what I might have done that could have offended her so much that I am not worth acknowledging. We have never had a conversation.
Given all of that, I'm operating on the assumption that the problem is hers and not mine. I've simply stopped greeting her or even smiling at her. I might make eye contact, but that's about it. Our church picnic is coming up next weekend. I will be my usual friendly self with the other women and not concern myself with Erica.