macOS Mojave installation trouble

Hello, I have a late-2013 iMac, running High Sierra, and am having trouble installing Mojave. I have plenty of hard drive space (over 700GB available).

I've downloaded Mojave from the app store, and the "install macOS Mojave" file is in my application folder (v14.1.0, 22.8MB, which seems WAY too small). The when the installer starts, I get through the initial process to the first restart, then the trouble seems to start. The restart is triggered, but when the iMac comes back to life, it's as if nothing has happened and I'm right back to my High Sierra desktop. No multiple restarts, etc.

I've tried downloading twice ... when it didn't work the first time, I trashed the installer file from Finder and did a hard restart then redownloaded from the App Store. Still no joy. Any help greatly appreciated!