When Some users reply to a manual notification from a ticket, their response is not added as a Log Comment to the ticket, while other responses are logged. Please evaluate
Release : 17.3
CA Service Desk Manager
Please check the email addresses that are being used to log the ticket updates. A common scenario that may cause the issue is that two or more different contact records were found to be using the same email address.
Service Desk will select only the first contact record it comes across that matches the sender's email address. It will not be able to select a specific contact record based on any other parameters, including contact type.