Radius freeze

Hi everyone, I'm using FreeRADIUS Version 1.1.7 with Mysql. I use for Hotspot system. All works fine, but after around one day without activity, the radius server don't answer. Therefore, if I restart (/etc/init.d/freeradius restart), all works ok!! I monitor with the command "ps" and just restart result: # ps ax | grep freeradius 26118 ? Ssl 0:00 /usr/sbin/freeradius Immediately after the connection of a hotspot's user: # ps ax | grep freeradius 26118 ? Ssl 0:00 /usr/sbin/freeradius 26158 ? Z 0:00 [freeradius] <defunct> What does this mean *_/[freeradius] <defunct>/_*? If hotspot's user disconnect, nothing changes!! Could you help me??