Just FYI your calculator's cost of living adjustment makes it completely uncompetitive for the Memphis, TN market. Possibly you are already getting good leads from other countries, but I wanted to point that out all the same.
I think the issue is their base salary is completely off.. Especially when the 'rent index' is based in NYC. A Junior Dev w/average experience would make 87k in NYC. A comparable government GS-13 position (average experience) in NYC would make 95k.. that 8k difference compounds a lot when multiplying.
Also I don't believe the rent index. Who cares where your employee lives (at least country by country) - if they want to live in the midwest, why not pay them the same as somebody who chooses to live in California, especially if you are an entirely remote company.