Insurance Question on Co-Pays and/or Deductible?

Doing a paper on Co-Pay/Deductible, my question would be, if a patient does not pay his/her co-pay for a medical bill, can the insurance company legally refuse to pay the remainder of the balance or their portion? If it is NOT legal where would I find the documentation stating that the insurance company has to pay the bill regardless. (keeping in mind that the patient's insurance is active)