You did not state why there was a denial. Diabetes would normally be covered unless it was present prior to the policy effective date. If it was present prior to the policy effective date, then it would be excluded from coverage, since the policy must be effective prior to any illness or injury.
Each company has a dispute resolution process. Look at that section of your policy under duties or claims filing to determine your next steps if you disagree. You can also call your pet insurance company to request a review or ask how you can dispute their findings.
Most policies require you to go through the internal review process before taking other actions. And to be fair, I advocate this process as anyone can make a mistake and this provides an opportunity for a different claims adjuster or supervisor to take a look at your claim and the circumstances for denial.
The reason for your denial should be spelled out clearly, and it should reflect one of the exclusions to your policy. For instance, if the company determined that the claim was pre-existing or present prior to the policy effective date it will state this as the reason. There must be evidence, such as prior medical history or medical foundation in order to deny the claim as being pre-existing. Some companies utilize independent outside veterinarians to review their disputed claims. Contrary to popular opinion, insurance companies do not want to deny claims and want to pay all covered legitimate claims; but cannot pay ineligible claims.
If the review is not in your favor and you are still firmly in the belief that the claim should be covered, then you should file a complaint with your State Department of Insurance. There is no cost to you and your state will make inquiry to your pet insurance company, gather the facts concerning the claim, obtain a copy of your policy and make a determination of whether coverage exists for your claim.
In most cases they cannot compel the insurance company to pay a claim, but they have strong influence since they regulate the company’s ability to provide insurance in their state. If the claim is clearly not covered, they will indicate that.
If after the internal review process and your state’s review you are still not satisfied with the outcome, you may try the company again at a higher level or seek legal action, which will be at your cost unless you prevail. If you win in court you may be repaid for your legal expenses.