If that happened with the same amount of flags, you either bought other (worse) flags, or you replaced good buildings with poor ones
Rewarded buildings might look good at your station, but they are not always better than the buildings that are available in the shop
To check if a building is better/worse than another building, take the amount of passengers/hour and divide it by the building's size (grid space) , the higher the number, the better
Example:
The Small Farm has 10 pass/h and is 7 grids
10 / 7 = 1,43
The Skyscraper has 62 pass/h and is 10 grids
62 / 10 = 6,20
As you can see, the Skyscraper is better than the Small Farm
