Scenario
When editing Lead Source on Lead/Prospect/Customer Record, not all campaigns are showing.
Solution
Lead Source Field on Lead/Prospect/Customer Record is based on the Campaign Category Field.
Campaign Category is set on the Campaign Record used for tracking purposes on reports.
If a certain Campaign Category is set on Lead/Prospect/Customer Record, then it will display Lead source that has the same Campaign Category.
To display all Lead Source, then unset the Campaign Category.