I have found a AP during a site-survey of which the a-radio was not up (you could not see the AP and could not connect to it), but in the WLC and WCS there was no error; everythings seemed to be ok (the a-band was UP in the browser) !!!
A reboot of the AP did not solved anything, I have replaced it with a spare one and the coverage in that area is ok again.
One could understand that, if an AP fails without telling you, you can have coverage problems without knowing why !!!
Is there someone who has an idea how we can detect this ? (logfiles, syslog .......)