Check the temperature. Snow will melt over time. Terrain affects snow too so you may see snow in some parts and not others.
DTG keep giving the temperature answer to this issue. That’s not a good enough answer. DTG put an option that puts in snow in one system and then made it melt with another system. It’s very poorly designed. If a player selects snow, it’s because he or she wants to drive in the snow. Therefore there needs to be a way to select snow and not have it linked to an average temperature of the region, where it will obviously melt, but have a temperature that will allow snow. It is a genuine issue and should be taken seriously by the devs when brought up by the players.
In the negative (Celcius). You can also start with no snow, turn up precipitation (in winter) and at some point you will have snow settle on the tracks and ground.