GPRS Shield send/receive Problem

I try to work with the GSMSHIELD Libary and i want to send and recieve sms.
My problem is that if I send a message with

sms.SendSMS( “+4932425425”, “Please send the Name”);

that after this i can not receive messages anymore with

sms.GetSMS(position, phone_num, sms_text, 100);

maby you have an idea, Greats Lennart