If someone on your team can't see a contact that should be in the inbox, it's almost always the first reason below. Work down the list.

Go to Settings → Inbox → Inbox Settings. The first setting, Allow Non-Admin Users to View All Contacts, is off by default.

While it's off, anyone who isn't an Admin or a Manager only sees the contacts assigned to them and the contacts that aren't assigned to anyone. So if a contact is assigned to Sally, John can't see it. Admins and Managers always see every contact, which is why the account owner usually can't reproduce the problem.

Turn it on and everyone can see every contact. Most teams do. Leave it off if you want each setter working only their own contacts.

If you'd rather keep it off, you can still hand a contact to someone by assigning it to them. Assign, Reassign, and Hand Over a Teammate's Contacts has the steps.

When a Spotlight or filter is on, the inbox only shows the contacts that match it. Two filters hide contacts people often expect to see.

Turn the Spotlight off behind the lightbulb icon at the top left, or clear your filters, then search for the contact by name.

Private contacts are hidden from everyone except the account owner. Contacts marked as spam are hidden until you turn on the Show Spam filter.

If the contact is missing for the account owner too, the message never reached Ellipsend. Work through Messages Aren't Arriving: The Connection Checklist.