Back to My Mac Only Works One Way
I may be jumping the gun here with my answer, but I'm going to assume that one of your Macs is running macOS Mojave and the other is running an earlier version of macOS.

macOS Mojave dropped support for the Back to My Mac feature. Instead, users are encouraged to use iCloud Drive, screen sharing, and Apple Remote Desktop for the various features you may have been using Back to My Mac for.

For more info on this change, refer to Prepare for changes to Back to My Mac on Apple's website.