About incursion and stone of passage

Ok I am getting really frustrated on incursion because I can't connect the rooms at all...

What exactly are the requirements for a stone of passage to drop in incursion? I haven't played this league and I don't get it at all. I have seen stone of passage drop in my 3 first incursions and ever since never one stone even when there is nothing left to kill? First of all is this guaranteed drop? or could it be that if you are unlucky you don't get any even if you clear the room? Does the drop rate depends on your magic find?

You are guaranteed a stone every incursion that has red doors so either you are not killing every monster or your loot filter is hiding it.

!!!
The loot filter, dang it I had not think of that

Thanks a lot

I have set up a special filter for stone and bingo I get the stone with no problem, thanks again

If you're using a custom filter and haven't set a minimap icon for it, I really recommend doing it, btw.