Insurance companies are investment companies looking to minimize their risk of exposure. They want to deny your claim or play you partially. A few reasons insurance companies commonly deny a claim are:

• Variations of the same story.
• Insurance companies will use multiple versions of the same story to cloud facts and distort liability and medical treatment.
• Can't Contact their insured.
• Insurance companies have a contractual obligation within their policy that states they don't have to pay on claims where their insured doesn't communicate or appropriately participate. They call this clause a “reservation of rights” and commonly use it to deny claims.
• Word vs. Word
• Sometimes the insurance company will indicate that they can't determine who's at fault because there is no other evidence outside of your word and their insured's word. Usually, this is used in a contested lane change. However, an attorney will go the extra mile and investigate the facts by looking at the scene, possible cameras and 911 tapes to track any possible witnesses to determine fault.
• Incorrect Information
• Insurance Companies will deny a claim due to incorrect information. Which is why it's so important to understand that the adjuster isn't out to make things right, but to conservatively save the company money and reduce financial exposure. They will use statements made against you, even if taken out of context.

What you can do about a letter of denial?

1. Accept it and do nothing.
2. File in small claims court if the damages are small.
3. Get a Lawyer

NOTE:
If your claim has been denied you will need to collect as much evidence as possible. The purpose of a proper investigation is to file lawsuit against the responsible party and bring your case before a Court. Be mindful that Courts have requirements for admissible evidence.