The Uptime calendar on asset pages is populated using data scanned from Windows computer's event viewers (power on/power off/sleep events). If you check the Scan Time tab of an individual asset page, you can verify if this item is currently being scanned, by default it is information that is always gathered during a scan.
If under the Scan Time tab the Uptime item is listed as scanned recently, it's possible that there is an issue with the event viewer on the local computers. If the item has not been scanned recently, its scanning may have been disabled, you can re-enable it under Scanning\Scanned Item Interval.
