Pain relievers and why they don't seem to work?

When I experience pain pertaining to headaches and muscle ache no form of over the counter pain relievers seem to work. Why is it that I can take take two to three 500 mg tablets or capsules and they don't seem to calm the pain at all? I have even taken up to five 500 mg pain relievers and they seem to calm the pain at about 85% relief wise. Could someone with medical experience please help me out?