Based on my observations, sims in committed relationships are VERY unlike to accept flirts, unless they have the flirty trait; you should consider this, too: maybe Nancy Landgraab rejects your flirts just because she's married.
This is correct, and seems to be even more the case with with the Awesomod installed. As your said, them having the Flirty trait seems to open the door to flirting somewhat, as does having the trait yourself. I believe that Charisma is also a factor, for obvious reasons. Their mood is also an issue, so try to flirt in rooms with absurdly high decoration values, right after they just ate a great meal, and/or have all the furniture buffed up by Mad Scientists. Anyone can be broken in optimal conditions, but optimal conditions often means you having the flirty trait, maximum charaisma, and them being in a perfect mood. That last part can be the hardests, as your target will often be coming over right after work and have very low mood bars across the board because of it.
However, being receptive to flirts seems to be a flag value because if you can pull it off just once then it quickly snowballs into a Romantic Interest easily. So if you
really want to get Nancy Landgrab and have the Awesomod, there is an obvious (cheat) workaround to this: Use the CTRL+click to select her, make her flirt with your character once, and then switch back. She's now flagged as being interested and will give a positive response to your own flirts, now.