Find my phone not working

Hello guys

My iphone 5 was recently stolen and I did set up the whole find my phone app.

The thing is that when I try to log to find my phone I got an error message saying No devices and that is impossible because when I go to my Itunes on my computer it remembers my iphone and it even appears as IOS making purchases in the Istore and stuff.

I am really really angry because my phone was stolen and I have the phone number of the guy that did it (He changed his whatsapp number before leaving a group) but I can't by any means find who does that phone number belongs.

I want to know why Find my phone is not working or what could I try to do to make it recognize my phone...

Thanks a lot