This was going to be my suggestion. I had a mare that was either perfectly content or walked non stop. It turned out that any negative experience on her first couple days somewhere set her off and she never got over it. Moving her to a different stall or paddock was an instant fix and she never thought about it again. She was never overly attached to her neighbors but if she was the first or last horse in or out before she settled in to a new place, that set it off again. Once she knew the routine, every horse on the property could have left for a weekend without her noticing but that “bad stall” set her right back to square one if I tried it again before she had 6 months to forget.
So… my advice is to switch stalls and manage her experiences very carefully for the first week. First one fed, middle horse for turn out/bring in, another horse at her end of the barn at all times, etc.