They are not responsible for a car that might have been stolen before the coverage began... In that sense, they did not accept that risk. If you listed the car on the insurance, and it was already stolen, this is your mistake. There is not an insurance company in the world that will pay for a car that was missing before the coverage period began.
That said, if the car was merely "in the shop" as far you knew at the time the coverage began, and only later did you realize it was never coming back, they should cover it.
I understand their issue. They want to know whether the car was missing at the time you insured it. If you don't know, I would say the date that he told you it was going to be somewhere, and it was not there, should work.
You might have to sue them. It is common for insurance companies to deny their own customer's claims. But when you sue them, they often give in and pay, because they can face hefty penalties for failing to pay.
If the car had any value, you should not have a problem getting a lawyer to handle it for a contingency.