Maybe nobody came down the stairs.
I was always struck by how fortuitous it was for the assassin(s) that Truly and Baker missed out the 6th floor on their way up the building. Being the cynic I am I came up with this crazy scenario -
Truly's role is to make sure the first police arriving don't go on the 6th floor so he stations himself just outside the front entrance steps. If none arrive for a few minutes, all well and good, no need to worry but Baker shows up in seconds. When Truly introduces himself to Baker he has already decided they are going to the roof (in his testimony he describes it as an almost psychic-like moment when he just knows where Baker needs to go). Truly takes control and leads the way. They get to the elevators and Truly can't call the west one down (or he only pretends to try) and the shouts to whoever is on the upper floors the equivalent of "I'm here with the police boys, It's showtime". Truly races up the stairs with Baker trailing behind.
Even though they are racing towards a shooter, civilian Truly leads the way, in front of a policeman with a drawn revolver. He has to lead the way because he has to control the situation but almost loses that control as Baker goes "off piste" on the second floor. Truly deals with that then gets them back on track. As they are on their way up the west elevator moves from the 5th to the 6th floor. On the 5th floor Truly leads Baker to the east elevator which they both jump in. On the way up to the roof they pass the elevator with the assassin(s) in.
Once they are on the roof the east elevator makes it's way down to the first floor, Dougherty drops off whatever or whoever he has to drop off, then returns to the fifth floor.
The only people on the stairs, in order:
Adams and Styles race down to see what's going on
Truly and Baker race up to the roof
BRW, Norman and Jarman head down to first as no elevator is available.
It's great to let the imagination run wild.