There are a few steps that need to be checked for this issue:
If the issue is for all emails coming to your support email address, check if the forwarding rule has been set up as shown in the link. Only if the forwarding rule has been set, would emails from the mailbox get forwarded into Freshservice as tickets. You can check if the forwarding address is working by sending an email directly to the forwarding address displayed under Admin > Channels > Email Settings and Mailboxes.
If the 'From' and 'To' addresses of the email are the same, it will not get created as a ticket in Freshservice. The same applies if the 'Reply-to' and 'To' addresses are the same.
Please check if you have enabled Service Desk Restriction (only emails from specific domains can be created as tickets). Move to Admin > Channels > Support Portal Settings and scroll down to check the same. If you have chosen 'Users from specified domains', change it to 'Users from any domain' and save the changes.
Go to Tickets tab > click on the view name (List) > select the Spam folder > check if the ticket is present here.
If so, restore the ticket > click on the Activities at the top right corner and check if there are any automation (Ticket Creation, Time Triggers, etc) that have worked on the ticket or marked it as spam. This is to ensure that future emails come through as tickets.
If not, click on the requester's name displayed to the right of the ticket > check if the requester is deleted or deactivated. Reactivate the requester and all future emails from the requester will come through as tickets.