A number of factors could be preventing you from receiving notification emails about comments:
1. The client did not review the entire revision. Revisions must have Approved/Changes/Remove applied to every spread, including the cover, in order for the email to be generated.
2. The notification is going to your spam folder. This is an automatically generated email, so sometimes your email provider moves it to the spam folder.
3. Your email address is not correct. Double check that all of the spelling and capitalizations are correct.