MacBook Stolen and locked remotely through Find My Phone but unable to locate it
Macbooks don't have GPS chips, so they have to rely on less accurate information - namely, triangulating a position from nearby Wi-Fi's.

While that works great in many cases, there's no guarantee it will. If no Wi-Fi networks are in range, a Macbook may not be able to gauge a position at all.

If the E-Mail pointed out the IP address it was sent from (I've never used this service so I don't know how it works) law enforcement may be able to do something with that - but chances are probably slim, as an IP address can be used by many people at the same time.