is there a way I can get eBay to investigate?
eBay doesn't investigate deliveries. If it's tracked as delivered as far as eBy is concerned, it's been delivered.
In any case, if you used eBay's new simple delivery any "item not received" case the buyer opens will be eBay's responsibility, not yours.
If the buyer paid by credit or debit card they may have more chance of a refund through their card issuer. Especially if this was bought from a business seller. Chucking it in a flower bed is not "delivery to a safe place", especially if it's within sight of the road.