Either water in there or bad fuel....
All the gas stations in cold areas switch to winter fuel specifically so you dont have to worry about it gelling. You shouldnt have to add additive to it.
In the 200k miles ive had my truck, between up at school in upstate NY for 4 years, and over a month of solid below 0* temperatures this winter in Wyoming (coldest so far I think was -20* or so), Ive never had any issues with fuel gelling. And I deleted my stock fuel filter head too, so I dont have a fuel heater either.
I dont run any additives or anything, I just fill it up with whatever they serve at the diesel pump.
ben