It's possible that she was scolded for going potty in front of her owners in her previous home, so she's simply scared to go in front of you. Try letting her go out on a long line and ignoring her while she sniffs around. Some dogs don't like going on leash, so giving them so room helps. Just sit in one spot, don't do anything exciting. If she goes, gently praise and reward (unless she likes when you get excited, but if she's timid about potty time, then gentle).