Not Receiving Replies on Support tickets

Two of my agents have been sending responses to customers who have been sending in tickets, and the customers have been responding, but they are not being picked up by zendesk and being brought back to us. We've received confirmation of these responses via a third party, however they don't seem to be showing up on the actual ticket themselves. We've identified this with two individuals and have double checked that their access is not suspended. Responses to tickets were confirmed on the 17th, and 23rd of june which does not appear to be occurring around any kind of downtime.