When making a claim of personal injury it is generally accepted that you will provide or find ways to provide proof of injury and that it is directly related to those you are blaming for the claim. Insurance companies will do everything they can to trip you up with such evidence and required documents as there is a good chance this will be a way not to pay you out. A good place to start is to get as much evidence as you can written down and signed along with any video/ audio evidence that can provide solid proof of injury.
Your claim may also be denied if you suffer from a preexisting condition. The Insurance company will try to argue that your condition caused your injury rather than their clients accident. This is very prevalent with tissue damage claims, including muscle pain. These type of tactics are employed to try and redirect blame onto yourself, the best way to combat this is to not dig yourself any deeper into the issue by giving them your words to stand on. It is very easy to misconstrue your words to work against you even though it was not what you are trying to say.
It is very common for a business to have a contingency plan in place that may seem to be helping you as a victim of an injury but in the long term it can be devastating to your claim and could potentially void your case. For example, If right after an injury has occurred you are given a document that promises to pay you a straight $500 for your medical expenses right now, this should be alarm bells going off as further down in that agreement is a statement claiming that your will not claim any further from the insurance company as the money is the pay out.
An insurance company will play all their tricks to find a way to deny your claim including using how long it has taken you to start the process of making the claim against them. This is why it is incredibility important to get the ball rolling right away.
It is very common for insurance companies to try and gain access to your medical records if they find out you have previous injuries related to the current claim. They do this to try and mitigate and redirect the blame away from the accident that took place. Although you are protected under the classification of an "Eggshell Plaintiff" meaning previous injuries that make you more susceptible to injuries cannot shift blame, you can still have your claim denied if the insurance company manages to prove that the injury was from a previous accident.
These are 5 of the more common ways your claim can be denied and it is highly recommended you take these into consideration while or before dealing with an insurance company.