wpa supplicant can't associate the same ap again

[wang baisheng]

hi,
        My WLAN interface is ipw3935. I found a very strange problem.
        If this is the first time that I associate the wlan, evering thing is 
ok. But 
when I down my wlan interface and then associate again. The output 
is "Authentication with 00:00:00:00:00:00 timed out". But after I do the 
following operation : "iwconfig eth1 essid "xxxx"", here the xx is another ap 
essid, then I try wpa supplicant again. Everything is ok. Why ?

        Thanks in advance.
        BRs,
        Baisheng