Check the amount of raw inputs the city has. To be profitable, the trader needs to have an abundance of the input your workshop needs. For example, I put brewing workshops in places where the amount of grain was very high, like over 300 units. You can help this along by buying the items elsewhere and selling them to the trader.
In addition, I did notice that it can take a couple in-game days for the profit to increase from zero once the workshop is purchased.