Low battery volts. The fully charged battery should measure at least 12.8v static across the terminals. The "brain" has a predetermined cutoff voltage of approx. 9.5v. Voltage drop from the starter will pull the fully charged battery down to approx. 10v. If the battery was low to begin with there won't be enough volts left over to signal the "brain" to trigger the coil to discharge. "Brain" being the ICM.