Update, in case it will help anyone else - I don't think it's anything I've done, but the situation sort of resolved itself. I've made it a point to 'get to know' the boss mare and she'll let me walk up and pet on her if I don't have a halter. Today, I was able to retrieve my mare with zero drama - So anyway, sometimes I guess I just need to have patience. I did carry a crop when taking her back into the pasture because boss was at the gate, but I just showed it to her and she walk off. We'll probably have good days and not so good, but at least I know this isn't some big thing.