I had a problem with followers not following me into a worldspace a while ago which was cured by setting the mod to be a master.

Now I have a similar problem, occurring even if the mod is a master. In this case it's an interior cell, and followers may or may not follow me in there. There does not appear to be any pattern to it. SCC followers are more reliable at following me in there than the standard followers, but it's still hit or miss. There is nothing special about the cell that I can see, none of my other interiors causes any problems. If they do come with me, the followers appear to have no problems of any kind in the cell. Unlike the previous worldspace problem, I am not actually losing followers this time - they are there when I exit the cell.

Has anyone seen this before? Any ideas how to fix it?

Did you 'Finalize' the navmesh inside AND outside? It sets the navmesh to green under the door marker so the AI knows there is a doorway there.

Yes, it's all finalized. The green triangles are present. That was the first thing I checked! I did notice the interior navmesh was very close to the door and did wonder whether the game was actually dropping the followers into the void, but moving it made no difference.

The odd thing is that SCC followers follow successfully maybe 80% of the time, but the standard followers only about 25%.

Intelligence plays a part in the ability to use doors. There may also be other settings or even scripts on the followers that 'follow' that you do not have.

Make sure you don't have any script on the doors with OnActivate. I recently had that problem and couldn't figure it out, because I had OnActivate and then only did something if the player was the activator. You need to activate the door normally if an NPC try's to interact with it, so

Begin OnActivate	If IsActionRef Player		...	Else		Activate	EndIfEnd

There's no script, but there is a trigger which is intended to ensure that the door remains locked unless the player talks to the owner. I wonder if followers will activate an 'OnTriggerEnter player' block? If so, that would explain it - I am unlocking the door, then they enter the trigger and lock it again before they can get through. Interesting!

Well, I got to the bottom of this, and it's an interesting discovery. What was happening was that when I teleported into my interior, I was just clipping a trigger box that was locking the door. It appears, then, that the game actually keeps track of the lock status as it teleports followers - if they were not in the cell before the trigger was registered, they were locked out! Thus the seeming randomness - it was entirely a matter of chance (it seems) which registered first: the teleport or the trigger.