There are a number of reasons why the iPhone may not be appearing in Find My iPhone. The iPhone may have been turned off, either deliberately or by having run out of battery, the iPhone may have dropped it's connection to the cell network and not be in range of a WiFi network, or the person who found it may have turned off the Find My iPhone feature or restored the iPhone completely. There's no way to know, but unless you get really lucky and it reappears - unlikely, but it's possible should someone just turn it back on - there will be no way to track it via Find My iPhone.
If the carrier doesn't offer their own tracking service, something you usually have to sign up and pay for, then tracking the iPhone will not be possible. . Contact the police and have your daughter change any passwords for any online accounts she may have used from her iPhone (the iTunes Store, for instance).