Receive data pause

Hi everyone, I use matlab transmit data to my microcontroller via Xbee S1, and sometimes the Xbee which connect with microcontroller( I call it Xbee Local ) connot receive data. The program on matlab keep transmit data about every 20ms, but the Xbee Local sometimes cannot receive data in a little time. I have recorded the state of serial port, and confirm it had transformed data, Xbee Local may not receive data. How should I do let my Xbees transmit and receive data completely?