We have a 6515 with the wireless adaptor installed on the device. When the machine is awake, it is maintains WiFi connection just fine. When the device goes to sleep, the WiFi is not automatically reconnecting. Rebooting the machine results in the device reconnecting to WiFi and working just fine until it goes back to sleep.
Is the firmware up to date?
Is Wireless set as the primary network?
Home > Device > Connectivity > Primary Network > Wi-Fi.