The USPS is under heavy amounts of stress. As of the beginning of 2021, domestic shipping times seem to have settled back into their usual rhythm, but still, some packages can take a while to reach their destinations. If a package is late, the tracking will usually update to some variation of simply "In Transit" or "Arriving Late" with not much else of an explanation. In the vast majority of cases, this is nothing to worry about, and it means exactly what it says. Please understand that once we drop off your order at the post office, it's quite literally out of our hands. Now, if you're in the US, and your tracking hasn't updated in well over a month, then maybe that's cause for concern.
Update as of August 2022: Many USPS post offices in the US are no longer updating Media Mail tracking numbers. It's very possible that your order will show up without the tracking ever updating.
For international shipments, it's extremely common for packages to get held up in customs for weeks at a time, and for packages to not reach their destinations for months after shipment. Packages arriving upwards of 3 months later is very common.