Data loss while received from serialport.

Question

On sending the NC data from richTextBox using serialport including ASCII header and footer code. Data is received to the receiver but only a single word is missing in the middle line of received data.

data send
X300Y-1300
X610Y-1500
X720Y-200
data received
X300Y-1300
610Y-1500
X720Y-200

here X is loss from the received data.if i send 40 lines of data it will works fine, on exceeding the line number this error will occur .is it a hardware or application error??
please help.

data is send from pc to CNC machine
code:-
serialPort->Write(header + "\x25" + "\x0A" + "O00000010" + "\x0A" + " " + "\x0A" + "(from pc) " + "\x0A" + txt /*+ txt1 + txt */+ "\x0A" + "\x25" + footer);