x-bee is in out of network

Am using 1141(co-ordinator) & 1341 (end devices)
Actually am using 20 x-bee module. More than one x-bee module is going to out of network communication oftenly,then after power came or reset the x-bee it will join into network again. please give me the solution for this problem.
then how can we find out this is parent & this one is end devices??

You can use ND command to find out parent node of a module.
If it a moving network then it is difficult to maintain range, but if it is a stable network, you can place the XBee, in the range of parent XBee.