Why won't this seller respond to my cash offer?

For over a year I've tried to buy a property whose owner keeps relisting it, flipping between pending and active, and never responds to any serious offer, including a contingency-free cash offer with specific inspection requests. We have never met, so I can't attribute it to personal dislike. What's the established protocol for presenting an offer when a seller won't engage, and would a buyer's agent change that?