My guess on what's going on: First, you're a FSBO. Nothing wrong with that. But to many agents, that sends the following message: "Selling a property isn't all that difficult. It certainly isn't worth a 6% commission [note: commissions are negotiable; 6% is just an example] to pay to someone else for something that I can do myself. Heck, selling a property means putting it on some web sites, maybe paying a few hundred dollars or less to get it in the MLS. As for pricing, I know what some houses around here sold for, and I can check Zillow. I'll do a little work and save a lot of money."
I'm not saying that's what you thought. What I am saying is that this is the reputation that FSBOs have among some Realtors. So there's no real eagerness for them to help you out.
However, as noted below, if an agent has a client interested in houses similar to yours, they would be serving their client's interests by showing them your property. But if they don't have clients interested in your sort of property, they won't show it. Even if, as you say, it's priced below market value. Or if their clients are interested, but your property is overpriced, they will provide that information to their clients who probably will choose to see the lower priced properties.
I'm somewhat surprised you haven't had some agents approach you for a possible listing. However--here's this reputation thing again--FSBOs have a reputation not only for overpricing but for being stubborn "know-it-alls." THEY know how to price a property. THEY know how to market the property. THEY know how to do all the paperwork. That is, until their house doesn't sell and they decide they need a Realtor after all. That's the perception. And some Realtors don't want to deal with that. They'd rather work with clients who value their knowledge and expertise, and view them as working together to sell the property.
Again, I'm not saying that's you. I'm just saying that that's the perception that some Realtors have of FSBOs.
That may be part of the explanation for your experience.