Version 2.5.1 of Just checking is compatible with the following versions of Mac: OS X 10.8 Mountain Lion, OS X 10.9 Mavericks, OS X 10.10 Yosemite, OS X 10.11 El Capitan, macOS 10.12 Sierra.
On the official website, there is a beta version available as well, but nothing related to the availability of previous versions.
If you keep having issues with the software, then send an email to the developer.