Problem statement 4
You are using a data loader or Informatica Cloud (a Cloud-based tool to load data into Salesforce from a legacy system), and you don't see a field for mapping.
Debugging steps
This is primarily due to field-level security. The visibility of your field is set to false, and this has caused the issue. When field-level security is not visible, adding the field on the page layout will still keep the field invisible.