Escorts.

[IejirIsk]

I went to kill the Master. and decided instead of porting, to walk back... and got two escorts. worked out well... i cleared the floors... but odd... does that mean I can just walk on random floors up and down stairs til i get them all?

[stinkstink]

Escorts require a certain amount of open space to spawn, and can fail to appear due to closed doors. Sometimes you get an escort after returning to a empty level because in the process of clearing it you created enough space for the escort to appear.

[twas Brillig]

To expand on that slightly: escorts are decided (dungeon and floor) at game start. Then the lack of space can cause problems, most commonly in Dreadfell, Kor'Pul, and other dungeons with a lot of doors. As a rule, it can be useful to walk through any of those again in case you missed an escort.

[HousePet]

I think it might be the lack of a path to the portal that blocks it.