So back on Windows 7, I had no problem getting mIRC to flash on my toolbar when there were new messages or other activity on my connected servers.

On Windows 10, I have the same options active, but it's not doing a damn thing.

Anyone got any idea how I can make it work?

Join the club. Which mIRC version were you using under win7 vs win10?

See this thread.

Even though I can't get the newer v7.x versions to flash correctly under win7, but was getting 6.35 to flash at the same time. Even though different mIRC versions are using the same API, and have different behaviors, it's an OS-dependent problem.