As more_CHOOS stated, there are 2 tracking systems, and sometimes there's a delay when a package crosses the border. I'm a seller in Canada, and I've shipped to the US many many times. If there is a "guaranteed delivery date" on the system, the day after that date, if the package hasn't shown up, the seller can call Canada Post to follow up.
However, if it shows up that it has arrived in the US, the delay is USPS, and it may be US Customs, or it may be sitting in the corner on a truck. I've found that in a couple of instances, when there is no "movement" on the website for updates, the package has shown up within a week... Just long enough for everyone to panic a little, unfortunately. I would wait just a little longer... Inevitably, when I've initiated a claim on an unaccounted for package, it has shown up that very day or the next.