yes acne on back is slightly resistant to treatment also they could be some sort of superficial fungal infection. You can use benzoyl peroxide based soap like acnebar/acnedor/brevoxyl creamy wash during bath over the back along with epiduo gel once at night. You can also take tab fluconazole 400 mg stat single dose to cover your could be fungal infection. But I suggest to take mediction under supervision of some dermatologist.
Dr Prakash Khute