Hi Cheryl,
I'm afraid that there is no setting to force users to tag someone into an update. However, one setting you could consider is the 'Someone comments on a thread I am in' configuration (located under Preferences > Communication in users settings). This will ensure that if a user was included in the thread at some point, but is omitted from being tagged into future updates in that thread, they will still receive a notification.
Appreciate that this isn't exactly what you were looking for, but this setting might go some way to minimising the risk of notifications not being sent.
Best Regards,
Rich.