Iphone is not installing apps after restore
I got my iphone 3GS replacement ( a brand new set) today for some defects in my old phone. Now I used the backup in my itunes to restore the new 3GS (after updating the ios 4.2.1). Everything went fantastic, all the sms-es, even the last calls made, contacts came back, but the trouble started when I tried to install a app/ringtone! It's not responding! It doesn't install any more apps other than what it had after the restore. Can anyone please help me?? I am confused!
macbook pro 13'inch, Mac OS X (10.6.6), Iphone 3GS white, iPad 16GB