If problem with authentication, it is recommended that you check the log on the RADIUS server.
If you could not find any informations in RADIUS, you may have a problem with WAP.
With some customer networks, this problem occurred when too many mobile device is connected to one WAP.
Even if the user has not suddenly increased, mobile device remains connected to the WiFi, this causes depletion of WAP Wireless resource.
We avoid this problem by power off the device or turns off WiFi that is not in use.