In kitchens, grease buildup is the usual problem and in bathrooms it's hair. Garbage disposals are always trouble makers. First ask yourself if just one drain is a problem of if others near it aren't working either. If none of the drains in an area work, but other drains in the house are OK then the problem must be in the line after it comes together under the room, but before it goes into the main line under the house. If none of the drains in the house work, the problem must be in the main line.
Frozen drains can be a problem in cold conditions under special circumstances.
Normally, the volume of water from showers, toilets etc. and the fact that drain lines empty quickly means that ice buildup is not a problem, even in severe conditions. Therefore, drain lines are not normally insulated.