If a submitted claim contains errors or missing information such as patient or insurance details then it will result in the claim getting denied.
When certain services or procedures are not covered under a patient's insurance plan, they are not eligible for reimbursement according to the patient's insurance policy resulting in the denial of the claim.
The use of incorrect codes, such as incorrect ICD-10 codes or CPT codes also leads to claim denials as the service or treatment code used is wrong or different from what was provided.
If the insurer believes that the procedure or service provided to the patient is not medically necessary then even in that case the claim gets denied.
Submitting the claim twice or multiple times for the same service or procedure is considered duplicate billing and can result in claim denials.