The Council was somewhat incompetent.
But maybe it was also a cultural thing. I believe Kendra mentions something about her family seeing her being a Slayer as a great honor, whereas Hank and Joyce would be more "get lost crazy British people, we're going for frozen yogurt."
They could have bargained with Faith's mother though, who probably would have traded her daughter for a bottle of gin.
The fact they didn't try leads me to believe they were in the dark about the Potentials a lot of the time.