For the application at hand, the 2 relevant datasheet specs are the max allowed Gate-Source voltage which is 20V. I believe you said you only had 14.4V DC so you're good there. The other spec is the Threshold Voltage which is when the transistor turns ON. That spec is saying that if you apply LESS than 2V, the switch is OFF for sure. Apply MORE than 4V and the switch is ON for sure. Between 2-4V anything goes (depends on the time of day, phase of the moon, etc.). So applying +12V between Gate-Source will turn it On for sure. That's why I had you confirm the Gate voltage was toggling between ~0V and ~12V DC. There is method to my madness! 
