why won't the imessage work?

I have updated my ipod touch 4 with the io5 software. I have the imessage turned on in my settings but am having issues still. I can send a text using imessage to one person whom has the ios5 on his iphone. I cannot send a text using imessage to one who has ios5 on the ipod touch nor can I receive texts in imessage on my ipod touch. I have read through the manual and still can't figure out what is wrong.