State of Charge Specific Gravity Voltage
12V 6V
100% 1.265 12.7 6.3
*75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0

look about halfway down to see a better chart of voltage = amount of charge left.

You may have some voltage.. but you are not pulling enough current. Thus no start.

Replace your battery.