Solution 1: Make sure that the user has the correct permissions. If the user is an IBO or a Technician only, they will only be able to see orders assigned to them specifically. If the user has Referrer permissions, then regardless of their other permissions, they will only be able to see orders that they are listed on as the Referrer. Instructions for setting user permissions can be found HERE.
Solution 2: Make sure there are no filters set in the search options that are restricting the user's list view. A search tool overview can be found HERE.
Solution 3: Make sure the user is in the correct list view. For example, if the user is looking in the Customers list, they will not see a person who is a Lead but not yet a customer, and vice-versa.
Solution 4: Make sure the user's 'ME' link is set to the correct rep record. Instructions HERE.