Since they are working for a percentage of the amount of salary they have negotiated for you, they have a diminishing return to spend time pushing too hard. Add

Since they are working for a percentage of the amount of salary they have negotiated for you, they have a diminishing return to spend time pushing too hard. Additionally, if they fight too hard, too often, the company is less likely to engage with the recruiter in the future, if they feel they are overpaying for that recruiter's candidates vs another's.