With the Gmail integration, there are a few ways that would prevent emails from syncing over:
Your authentication token has expired (due to time since connecting, or you reset your password)
The email address from the email to the contact do not match up
The contact is using an alias email that is not listed in the contact app
Authentication Token
If emails have stopped coming over for all of your clients, your authentication token has probably expired. To fix this, simply go to "Your Settings" > "Email Integration". Click "Disconnect", and then re-connect your email.
After you are re-connected, emails from the past 24 hours will be pulled in as well as anything moving forward. If you need emails to come in from the past, please reach out via in-app chat with a specific timeline (ex. Past two weeks) to have a hard sync ran on the emails.
Email Addresses
For specific missing emails, the first thing to check is to see if the responder/sender email is reflected correctly to the contact object inside of ClientSuccess.
How to Check for Alias Emails:
Inside of Gmail, go to the most recent message and open it
Next to the reply arrow in the top right corner, click the drop down
Select "Show Original"
Inside of this original you will be able to see what email address sent the message and what email address received it. Check these email addresses to ensure that it is not an alias email address
After running through these steps, if you cannot find the reason for a missing email, please reach out via in-app chat or emailing support@clientsuccess.com.