Why can't I spawn my third harbinger?

I beat my first harbinger in normal monolith. I then move to empowered monolith and fight the boss, then there's a check mark to spawn a harbinger. I fought it and won. Now I play another empowered monolith, get enough stability for the boss, but the check mark to spawn a harbinger is disabled. So how can I enable it back? I tried fighting the boss and won but no harbinger is spawned. This happened twice already: fought the boss but no option to spawn a harbinger.