Sims get stuck

I kept having the same issue on my build with my sims getting stuck under the stairs during live mode, it was always the sims I wasnt controlling at the time that got stuck. 

I found it was caused by two things - the stuck sim wanting to autonomously use a water lounger that had floated off the boundary line of their lot, so they couldn't use it as the game didn't recognise it belonged to the household and behind the wall under the stairs is the bathroom. The one tile wall jut out behind the bath where the stairs join above was creating a "dead space" so sometimes the sims would fall down into that space when walking down the stairs. Since removing the wall and making and all floating loungers none of my sims have got stuck under the stairs. So it might be an unaccessible item or object, or a "dead space" near the stairs that is causing issues for you 🙂 (in the pic below you can see the problematic wall behind the bath that I have since now removed and I also moved the stairs outwards by one tile to flatten out the bathroom wall below where the stairs join the room above) 

Diablohorns​  The reason that happens occasionally is because the sims were traversing the stairs at the time you saved.  Sims can't be saved on the stairs, so what happens is the game places them on a clear tile close to the stair, including at times, in inaccessible spaces like a roof or under a stair depending on if MOO is on or off.  So, one way to get a sim out of an inaccessible space is to travel.   Another way is to use the cheat to teleport the sim.  However, that cheat will disable trophies on the consoles as testingcheats has to be on (then you shift click on the sim to get the debug and cheat options).    

Hope this helps.