These things happen with them. "Technically" USPS doesn't have true tracking unless it's express mail, otherwise they consider it delivery confirmation. Sometimes you get the added benefit of seeing where it is along the way, sometimes not so much. I've had packages drop out of sight completely after leaving the sending facility and then suddenly appear on my doorstep three days later. I wouldn't start worrying until it's been out of sight for 5 days from the date it was mailed.
This has since changed, actually. Now USPS only has one service: "Delivery Confirmation / USPS Tracking"
They've bundled it into one after they've realized that they are really the same thing.