RF Hang!! Plz help

Hi All
I get a problem with XBee modules occassionaly. I have some routers as well as one fixed coordinator in my network. Routers are installed in vehicles. and so the may be away from coordinator and may be close to it, Depending to their location. Some times when a router come close to coordinator the router is not recognized in node discovery command(ND) of coordinator!! But most of the times they are recognized!
If we do a reset pulse on XBee module which is not recognized by coordinator the problem will immediately removed and XBee will be recognized immediately.
Can u help me to find my problem?
Thanks
PS: XBee uart interface is running good in this fail state and it seems there is a problem in RF part so I write the title of the problem as “RF Hang”