Your machine only "machine authenticates" if it is at the ctrl-alt-delete screen. There is a timer in in the 802.1x profile under advanced that says how long the controller remembers that a machine has "machine authenticated" after being at the ctrl-alt-delete screen. It is the "Machine Authentication Cache Timeout" parameter and by default it is only 24 hours, so your machine would have to be at the ctrl-alt-delete screen every 24 hours for the controller to know that it has machine authenticated.
You can find this timer by going to configuration> security> authentication> l2 authentication> 802.1x profile. Find the profile that corresponds to your WLAN and under advanced, the "Machine Authentication Cache Timeout" parameter should be there. Extend it as long as you need, so that your users do not have to be log off then log on again.
