I'm having a few problems with followers and doors. I go through a door with multiple followers and not all come through with me. The things I know of that will prevent door use are:
The actor has insufficient intelligence to use a door - can't be this, the blonde is one of those who gets through.
A script on the door that will activate it only if the activator is the player, or selected NPC's - none are excluded here
The door is locked, and the actor tries to use it in that state without the key - this should never affect a follower, as the player will have unlocked it first.

Am I missing any other factors?

Could be not enough space for multiple NPC's to enter the entryway. It may be dependant on the navigation markers you have in the room.

I thought I'd found it, the two followers who wouldn't had low-level processing off (which would mean they'd stop being processed after the player left the cell), but that didn't fix it. However, I haven't gone back far enough to make sure it's not a save game hangover, so it could still be the answer. I'll report findings once I have a definitive answer.

It's not a space issue, as the same followers do or don't pass through regardless of their positions before, and the door leads outside where's there no constraint.

Yeah, with NPC's they would be 'saved' in your save game and any further changes to them would not apply to them. Go back to a save game from before you added them to your save-game.