Not getting what he had come for, Andrew returned to Comers Palace a little dejected. Obviously, he hadn't expected to win Cassandra's heart in a single day or get her into his bed, but he had to admit that the whole thing was a disaster.
Cassandra isn't a simple woman like the two textile merchants from before. While these two women are strong and determined merchants, they're no match for Cassandra.
The first two could easily fall for Andrew's charm, as evidenced by the fact that he managed to sleep with them at the party when they had barely met. That's why this time, when they came to negotiate, he had no problem using his skills to have sex with them again.
However, Cassandra is different. Not only can she resist Andrew's charm, but she's not frivolous like the other two. For this reason, Andrew was reluctant to use his skills against her in order to get her into bed.