Typically when YOU are not in person importing a vehicle/trailer from Canada to the US you get an hired border "agent" involved to act on your behalf for the importation of the vehicle/trailer. This "agent" is appointed by you and acts on your behalf as the importer. You said you are using a commercial shipping/transport company for the process of shipping the trailer to you. The shipping company should be able to fully explain this process to you, why the shipping/delivery is taking so long and why the trailer would be held in Customs for such a long length of time. IF they are not able to fully explain this to you why the trailer has to sit in customs for so long I suggest you are dealing with the wrong shipping/transport company without any experience in this process!
All this assumes that the seller does not want to get involved as the person importing the vehicle/trailer to the US for you. Every now and then you get lucky and the seller will take that responsibility
Remove the word "commercially" from your vocabulary as it relates to the importation of this trailer since this trailer was sold
to you personally and for your personal use. Say the wrong think to the wrong border agent and it just might cost you "Mo Tax Money"!
As Donna said her trailer beat her across the border. "I" think you might have a lazy or slow shipping company who is blaming their lack of ability to deliver your trailer to you in Colorado in a timely manner on the US customs agents!