There can be a number of reasons for this.
Firstly please ensure your mobile data is turned on.
Please go into the settings for our app on your phone and change the setting so the phone alerts are on (we cannot advise the exact way to change these settings as each phone is different).
This can also happen if there is two or more people logged into the same app account (using the same login details). In this case it may only alert one user and not the other.
2 or more people using the same account can cause various conflicts. We always recommend each person wanting to track the device set up their free app account and add the watch to their account.