Could it be due to capacity limit of the trading ship? A ship can only carry 12 units of all goods, so if like a city buy 3 typed of goods from you, they may buy 4 unit of each type per ship.
Would this be a problem? If thats the case, you can test it out by setting only selling luxuries and see if the limit can be reached.
[This message has been edited by XW2000alpha (edited 04-22-2020 @ 10:15 AM).]