Why does deodorizer lose effectiveness faster in winter cold?

I've been servicing a construction site with 20 standard units for three weeks now. It's winter, temps in the 20s, and I'm noticing the deodorizer seems to fade faster. Units that were fine on Monday are smelling by Wednesday, even with light use. The ground is frozen gravel, truck access is good, and I'm dosing the same formaldehyde-free blue liquid as always. In summer, these same units hold up fine on weekly service. Is the cold slowing down the chemical action, or is it something about the tank itself? Does the deodorizer just not vaporize as well when it's freezing? What actually happens to that blue liquid in the cold?