Randomly you will not see any results (or a very small number) in the Assigned To field on incident problem or change.
Sometimes this is resolved by clearing the cache.
If you have duplicate itil roles, this can affect the query made on the Assigned To field reference lookup or typeahead search and cause it to only return the users associated to the duplicate itil role.
Remove the duplicate roles. You will need to create an ACL on sys_user_role to do this because the OOB ACL prevents deleting some OOB roles.