I had the same experience (an expert of an industry had an idea about a industry problem that "desperately needed to be solved", so I got excited and then built a solution).
After the solution is built, the expert decided she's "too busy" to return emails.
It happens all the time. Experts have ideas because their 9-5 job requires them to have ideas. They have to push hard for ideas so they will be taken seriously at meetings at work. That's why they are "experts". It's natural for an expert to "push" for her own ideas--otherwise she can't get buy-in from the rest of the organization for their ideas/initiatives and won't thrive.
The quality of the idea is not ass important as the act of pushing for it. Because if an idea does not get support, it's useless, regardless of if it was good or bad.
The moment you ask them to be use your product, they cease to be an "expert" and is now a "user", or even a "paid customer". Suddenly, they don't find the problem so "major" anymore. So you have to really qualify them before diving into the problem. Really make sure they are committed as a user before you start building anything.