Seems to me like you have an underpowered DC-DC converter, or, that your voltage sag is so bad that it suffocates the DC-DC and doesn't let it produce enough voltage.
Put a voltmeter on your 12v and see what's happening. If it's plummeting, thars yer problum.
Put a voltmeter on your 12v and see what's happening. If it's plummeting, thars yer problum.