Bob
Probably the additive was not adequate to the temperature drop. On these chemicals, they usually work at a certain concentration level, but adding more might not buy you any protection. Different chemistries may give better protection at these low temperature levels, with certain fuels. It is not an exact science but is usually optimized for a particular fuel and expected temperature levels. The diesel fuel supplier adds it, although you can buy different additives locally, and they may, or may not, be effective.
At some temperature level, all these chemicals may prove ineffective, and the only remedy is to decrease the waxy components of the diesel fraction at the refinery.
At any rate, these are the trials and tribulations of living in 'cold country'
Best regards