Homeguard should trigger as soon as you get into the big circle of the fountain

I know what you mean! I don't understand the science behind it. Sometimes it is instant sometimes you gotta fiddleydiddle and walk around till the fiddle diddles your boots.

Sometimes it's because you're in combat (defending base), but yeah; sometimes it just doesn't work for seemingly no reason.

I wonder whatever happened to "home field advantage" they were going to add. It was a permanent MS buff while inside your base. It was mentioned to be added with the summoners rift VU. Never happened though.