At 12.6 V your battery is weak. I dont know why 12 V is not enough but for some strange reason a fully charged 12v battery will read 13.2-13.4V. I know there is not a lot of differance between 12.6 and 13.2 but it is enough to make the battery weak enough to not start. As mentioned before take the battery to a auto parts store and get a load test.