We were checking your backend setup, you have 2 cars available from 04:00 daily and we could see 2 cars correctly if we choose any datetime after 04:00. FYI, system will always check next available car if no car is available on the selected date time. But, if system still can't find any next available car, it will show No vehicle warning message.
We could not replicate the problem on your Booking Form, please check again and let us know if you still find any problem.
Thanks a lot