Xbee not going to pin controlled sleep when used standalone

Dear Sir/Madam,
I am trying to use Xbee S2C (XB24C) module for I/O line passing along with pin controlled sleep mode. It worked fine when connected to XBIB-U-DEV module but as soon as I tried to use xbee in standalone mode the pin controlled sleep function stopped working, however I/O line passing still worked.
I found out that it goes to sleep mode when its UART is connected to other module (e.g XBIB-U-DEV’s ftdi, Arduino etc) but as soon as I disconnect UART and start working standalone the sleep function stops to respond. PLEASE HELP ME TO SOLVE THE PROBLEM.

This sounds like the sleep request line is not being driven properly or one of the other lines are not properly connected.