I am having the same problem .
As Ajish told check if the right commands r comming out of ur AVR
download realterm from d link
http://sourceforge.net/projects/realterm/
go to display tab and check the ” Hex+Ascii” to see the exact hex value u r sending out
and do the same for SIM900 i.e send ur AT command in “Hex+Ascii” and check if both are same
I have done all dis and everything is fine but still my SIM900 is not responding 
I am finnding out next steps for troubleshooting