unable to send a string in sms using GSM

String str1 = String(flat,6); // float to string conversion
String str2 = String(flon,6);
str1 +=" ";
str1 += str2;


This is the part of the code im facing problem
ive used <GPRS_Shield_Arduino.h> library.
message is being sent but its is either blank or dummy text
when i serial print (str1) it is showing correct string but i am not getting same string as sms insteat i am getting dummy values
also i dont know why the no. 6 is used in last line but if i dont include it, it throws an error.
if any1 knows the syntax please care to explain.