If they seem to be leveling slowly, then you must retreat back a bit to level them, as , for instance, a level 20 merc wont get hardly any experience in act 5.
Once they are close to your level, and level 20+, they dont take near as much care except to rez when they stumble into fights they just cant win. (No, Jeeves, dont attack whilst you got Iron Maiden curse !!.....) Aieeeeeee