Videoloupe for Mac (https://www.videoloupe.com)
These are both macOS applications that have yet to reach enough in sales to be sustainable. However, I work on them full-time in hopes of reaching sustainability in the near future.
"Why do you think it hasn't been as successful as you thought it would be..."
Exposure and importance. Getting exposure for a macOS application (or any application for that matter) is tough. The lion's share of excitement these days is around mobile applications and web services. Trying to get publishers excited enough to write about a desktop application is challenging...
Importance is something that's taken me a little bit to understand. There might be some "utility" or "nice to have" applications for macOS that make a decent living, but I think if you really want to turn an application into something that has long-term sustainability, then you need to find a way for your application to become essential to a user's workflow. Excel, Lightroom and Final Cut Pro are all essential applications to their respective user's. Fileloupe and Videoloupe are "nice to have" apps in their current versions. People enjoy them, but they aren't essential.
With the exception of maybe a few outliers, I'm not convinced that you can make a living selling macOS apps for $10 and hope to make up revenue on volume. I think you need to get into a higher price range and if you want to sell a more expensive product to someone, then it has to fall under the "essential" category and not the "nice to have". That's my goal with Fileloupe 2.0 and Videoloupe 2.0, but there's a ton of development to do.
(Oh, and I'd forgot how much more work went into making a macOS application versus an iOS application. To say that my original time estimates were off is a laughable under-statement.)
"How much time/money did you spend building it..."
Development on Fileloupe started in the spring of 2014 and version 1.0 shipped in the summer of 2015. Development on Videoloupe started in the summer of 2016 and 1.0 shipped in the spring of 2017. Living expenses have been cut drastically over the years so that I can continue to do this full time. (Hint: Living in Thailand is a lot cheaper than living in San Francisco or Vancouver...) Sadly, I don't belong to any special "comma club" so there's an end-date to this dream if profitability cannot be reached.
"What kind of iterations / improvements did you make to try and salvage it"
I learned a ton building version 1.0 of each product. Arguably, I learned more than I should of and likely would have been better off starting with a bit more of a plan and clearer vision for each app. Regardless, I now have a much better idea of what each app should be and I'm hard at working on version 2.0 for each.
