Inconsistent data transmission with XBee

I am using XBee S2C (not pro). XBee+PC as a coordinator and XBee+Arduino as a router. I am using XBee Arduino library.
I was successfully able to communicate between them. Now I am finding that this successful communication is very unreliable at its best.

Suppose the communication is working fine. Then I stop the program. And start it again after a while, I find that they are not communicating. Resetting the XBee with the Reset button solves this 60% of the time. But not always.

What am I missing?

which antenna your using for application

PCB antenna, no external antenna attached

try to use 2 db external anteena