Something about your home is turning people off. They're showing up because your home (online and/or on paper) looks good--the right size for them, the right location, and the right price. Then they get there and decide they don't want it. There's something that's changing their minds.
You'll get some comments here suggesting your price is too high. I disagree. When people are looking at homes, they do a basic calculation before looking: Is this home in about the right price range for what's being offered? Is it in a price range I can afford? Few people are willing to waste their time looking at homes they think (before even arriving) are overpriced. They just don't do it. Most are serious. And you acknowledge that the ones you've followed up with have bought other properties. They WERE serious. However, they determined that there was something about the other home that worked better for them.
As for feedback being positive, that's almost always the case. I've looked at some real pits, and the agents have told me the feedback was positive. Why? Because most Americans try to be polite. That applies to buyers and to agents. They've been a guest in your home, so they don't want to say, "Gee, that green shag carpeting was ugly." Or "It smelled like a pack of wild dogs lives there." Or "I didn't know the house was at the end of an airport runway." Instead, they'll say, "I like carpeting." Or "We like pet-friendly homes." Or "Your home is really convenient to transportation."
Ignore positive feedback. Really. Hope and pray you get some negative feedback. Have your agent follow up with anyone else who views the home and ask, delicately, what they'd change if they could.
Suggest that your agent hold a broker's open to get feedback from other agents. That will sometimes produce some constructive information.
Consider having your home staged. At least get a home stager in there to give you a proposal. A reasonable charge for an initial evaluation might be in the $150-$250 range. The money spent will be well worth it. A good home stager will give you a good, honest, accurate evaluation of your home, as well as suggestions on what can and should be done.
And, of course, talk to your agent. Even if there have been lots of showings and generally positive feedback, your agent should have some suggestions or ideas.
Hope that helps.