Random responses from FTP server while conneting via GPRS

[javani_saeid]

Hi everybody
I wanna connect to FTP Server via SIM300 GPRS.

I am sure about my command sequence, but sometimes it works and sometimes it doesn't work!

Do you know what is the reason?

AT&F
OK
AT+CMEE=2

OK
AT+CIPMODE=1

OK
AT+CGATT=1

OK
AT+CGDCONT=1,"IP","mtnirancell"

OK
AT+CSTT="mtnirancell",,OK
AT+CIICR
AT+CIFSR

OK
5.122.78.243
AT+CIPSTATUS

OK
STATE: IP STATUS
AT+CDNSCFG="208.67.222.222","208.67.220.220"

OK
AT+CDNSORIP=1

OK
AT+CIPHEAD=1

OK
AT+CIPSTART="TCP","FTP.xxx.COM","21"OK
CONNECT220 Microsoft FTP Service
user xxxx
password required for xxx '

As you see, in this case, Server responses correctly, but here:

AT&F
OK
AT+CMEE=2

OK
AT+CIPMODE=1

OK
AT+CGATT=1

OK
AT+CGDCONT=1,"IP","mtnirancell"

OK
AT+CSTT="mtnirancell",,OK
AT+CIICR
AT+CIFSR

OK
5.122.78.243
AT+CIPSTATUS

OK
STATE: IP STATUS
AT+CDNSCFG="208.67.222.222","208.67.220.220"

OK
AT+CDNSORIP=1

OK
AT+CIPHEAD=1

OK
AT+CIPSTART="TCP","FTP.xxx.COM","21"OK
CONNECT220 Microsoft FTP Service
user xxxx
 '

Nothing is got from server!

what do you think?