Mindread the rebellious slaves and you will see something like Resistance (cant remember wording atm) and a number, usually up to 10
You want that number at 0, casting Fear is the easiest way to get it down, and it will go down over time anyway, though this may only take effect if they stay in jail (you can collar them and put them anywhere, but ive never seen a slave lose that tag when not in jail).
Since i just use fear i havent tested interactions much, but i assume maxing fear level using interactions would also help this process.
Its intentional, and actually scaled quite well imho