Can't detect sleeping XBEE

I’ve 1 Xbee Maxstream series 1. After setting the SP (Cyclic Sleep Period) in 10 seconds and ST (Time before Sleep) 1 second, I unplug it from the xbee adapter and plug into it again. Then I press the button “Test/Query” in X-CTU for numerous time but it shows the box “unable to communicate with modem”. At first I thought it could be caused by the damaged Xbee. Then I test it with other newly bought Xbee and other brand of xbee adapter and the same thing happen again. There was no such problem before I setting the sleep time of xbee. Can X-CTU detect the Xbee which is in sleeping time? How can I solve this problem?