Did you (your agent) get feedback from their agent? Any hint?
If they have seen the house 6 times, your agent made it very clear that you are flexible and your price is the best in the area, and still no offer from these buyers and won't give you a reason, I would not worry about those buyers and will just wait and let it happen when they are ready.
Maybe there are external reasons and worrying about it will not work - repair credits, waiting for funds to coming in (or savings), somebody is sick,etc. Any feedback at all?
Sometimes a buyer is never ready to take the leapt. With the market the way it is; makes it even harder. I know of a buyer who made three offers on three different houses throughout the years and backed out on all three, they had a reputation now for never ready - the wife and kids wanted to move but the husband is scared of a new mortgage. It just would never work for them unless they hit a jack pot, if ever it's going to happen.
Concentrate on the next buyer and who knows, a buyer will suddently show up - sometimes it works that way.
Now I said this, they may just show up tomorrow with an offer, so good luck!
Maybe the buyers are wondering what's wrong with the property if it's been on the market for 1-1/2 years. I know, maybe nothing's wrong with it. But still, that might be a question in their minds.
Maybe their definition of "reasonable offer" and yours is different. Let's say your house originally started off at $600,000 and, with your $120,000 price cuts, you're down to $480,000. Maybe your bottom line is $440,000. OK. But maybe they can only afford $450,000, and they think you'd be insulted with an offer at $445,000...another $35,000 drop from its current list price. So they don't think they can make a "reasonable" offer...even though they could.
Maybe they're having a hard time imagining themselves in the house. Not that there's anything wrong with your house, but you might consider home staging. Heck, after 1-1/2 years, I'd say home staging is mandatory. You want your home to be as inviting and attractive as possible...and you also want it to be as easy for buyers to imagine living there as possible.
Maybe they're not serious. Sad, but true.
What feedback has your real estate agent received from their agent? By now, those two should have each other's phone numbers on speed dial. Rather than speculating all day, why not actually find out what the problem is? (I'm not being critical...it's just that that's one of the many roles of the agent..)
Also, what other activity has your house had? If it's not getting many other potential buyers, then it may be priced too high. If it's getting some showings but no offers, then there may be something about the house that's turning people off. (We're back to home staging as a possible answer.) Whatever the problem is with the others (no showings due to price, no offers due to appearance) may be the same issue confronting your 6-time visitors.
Hope that helps.