Is there something in the existing room that would block the corridor if it were created? Bookshelves against the wall, that sort of thing? Even something moderately close to the would-be-new corridor can prevent it appearing sometimes. 
Darn poster who put that there?. And I found a post on US forums about stacking and now I'm trying it with just desks and it works fine. Cheers Standoff
Just started an Sg and want to create a new room in my base the instructions say wait for room to turn blue and click but it never seems to turn blue, even when a small doorway/ corridor appears and I think it is in the right place the room remains red and won't place. What am I doing wrong? anyone know
And while we are on the subject how do you place things on top of desks and tables?