Carriers schedules are constantly changing due to cancellations / mechanical issues / scheduling conflict and load availability. The volume of vehicles transported in the United States is higher than the amount of trucks / drivers. All carriers sub haul their work to other companies if they don't have a driver scheduled in an area. There is not one trucking company available to handle the volume of vehicles shipped within the United States. All carriers use the same load board to sub haul their work for a service fee.
If you're scheduled with a company and haven't received a response for a pickup or delivery date, than the company has not scheduled the driver. There are two main reasons a driver has not be scheduled. 1. A pricing issue, the cost listed to transport a vehicle is not the asking price a driver is willing to accept to transport the vehicle. 2. There is not carrier available in the pickup or delivery location. Some areas may be desolate or restrictive for trucks to drive, due to weight or height. If there is a problem locating a driver, you can increase the pricing or change the location to a major city.