Clarification of Conqueror influence and spawn rules?!?

Bearing in mind I have all 32 watchstones, and have fought (and beaten) Sirus on multiple occasions, I seem to have encountered what seems like a bug.
I have already beaten 3 conquerors but the 4th one (Hunter in Tirns End) will not seem to progress.
The zone is green, and my understanding is any map in that zone should have a chance to spawn him, but I've run literally about 30-40 maps and nothing has happened.
Is this actually plausible that you can go that long without a spawn?!?
Also, what are the rules with watchtowers disappearing? it seems like sometimes they are present in influenced zones, allowing for the removal of stones, and then other times they disappear. Again, is this a bug or are there rules that dictate when this happens?!?
Any insight appreciated, I'm starting to get pretty irritated at how long this is taking and can't help but suspect there is a bug or issue here.

This was my fault. The issue was that I needed to speak to Kirac, as the final spawn had already occurred and somehow this had escaped my attention. I guess this is also why the watchstone tower had disappeared. I'm an idiot.