Keep a record of everything you do to trace this person - include receipts for the adverts in the personal columns of newspapers.
If you haven't done so already put an ad in the local evening paper of the person's last known address and a national paper.
When eveyrthing that you can reasonably do has been done, and there is still no trace, consider Missing Beneficiary Indemnity Insurance. Such insurance doesn't have a time limit and will pay out if the beneficiary turns up. The insurance company will need to be satisfied you have done everything reasonably possible to trace the person.
Then the money can be divided between the remaining beneficiaries or given to the heir, depending on the wording of the will.