OP it was just cold. It seems like the larger engines have a harder time in the cold weather than the light duty engines.
My dad's 01 Cummins and my 05 Cummins do pretty good to about 10*F before they seem to have a 'no-fire' on #6 cylinder (assuming #6 since it is the farthest from the intake grid heater).
On my dad's '06 International DT570, it raises all sorts of hell near freezing and almost demands the block heater at 20*F.
On mornings you are having a hard time getting it started, cycle the heater twice. After the Wait to Start light goes out, shut the key off then turn it back to the ON position to cycle the intake heater a second time. It will suck in warmer air this way.
As for fuel additives, I run Howe's. Kept my '05 alive in Colorado at 11k elevation with -25*F temps, starting without the use of the block heater.