Here are the main reasons why no location could be provided for a message, from most to least likely.
Some specific message types that are emitted by devices will not be geolocated (e.g. OOB messages).
Station not contributing to the service
In most cases where no location is provided, none of the base stations receiving the message can be used by the location service. To confirm this assumption, please reach out to your local Sigfox Operator support.
Too many stations
If a message is received by a large number of base stations (i.e. > 200), then no location will be provided. This could correspond to high altitude devices or devices aboard a plane.
Stations too far apart
If two stations contributing to the same calculation are more than 220 kilometers apart, the calculation will be aborted, as it would induce too much uncertainty.
Data infrastructure failure
Since a cloud solution is used to host location algorithms, failure can happen. In such a case a replay mechanism is implemented, making the absence of response extremely rare.