1) What are the filters from your old board and your new board?
- It could be that the new board only shows specific issuetypes, excluding 80% of your old tickets, for example.
2) Do you have full permission to see all the issues?
3) Do you have any active filters on your new board?
4) Can you use a query to see all your issueswith Project = "projectname"? This will determine if your tickets are really gone, or just not visible.