Why would a medical insurance company deny a insurance claim 3 reason?

The 3 reasons why an insurance co. can deny an insurance claim are :-

1) Pre-existing disease not disclosed at the time of taking the health insurance

policy.

2) No Intimation notice regarding admission to hospital/nursing home submitted

to the insurance co. or TPA immediately on admission of the patient.

3) The papers submitted show that it relates to check-up and not for

treatment of the particular admissible disease as per policy.