Car insurance company denies my claim.

There should be some damage evidence in the rear of the pick-up that would show he was hit.  If the insurance company is giving you a hard time, ask for the proof that there was such damage.  If you are not satisfied with the insurance company's response, consult an attorney.  The attorney consultation should be FREE and you should get some specific answers as to why the insurance company will not accept liabiltiy.

KPS

Sounds a little fishy to me but if he can prove that he did have damage to his rear bumper then you can make a claim under your uninsured motorist provision of your policy because the other guy was a hit and run and there was physical contact between eh vehicles which is required under California law. If you were injured then you should see a personal injury attorney like me who handles cases on a contingent or percentage basis. As with all good personal injury attorneys your initial interview is free. The questions I would ask are again were you hurt? How much property damage was done to your vehicle and did you have car insurance. If you were hurt sought medical treatment within 4 days, you have more than $3,000.00 in property damage and were insured at the time I would be interested in discussing this will you further.

If it is just a property damage claim try to get your insurance to handle it and demand proof of what the other guy said i.e. photos of damage to rear of his vehicle and repair estimates. Advise according to California law by an attorney licensed in California