Was doing a greater rift 60 and finished with enough time but the rift guardian (The binder) failed to spawn.
I don’t know the exact cause of it, but I somewhat know the solution for it. Try going in and out of next or previous zone/level. When you come back to the level where RG is supposed to spawn, RG will port on top of you, just as they do when you go too far away from them.
he spawns, you just need to find him cause sometimes he can spawn on other side of the map lol or maybe he is behind closed doors and can’t spawn to you. 
Did try this and got nothing, but if it happens again ill give it another shot.