Stair problems, help

It's been about a year since I last played. When I last played this was not a problem. The problem now is stairs in buildings that worked before are not working now. I don't have any mods and my game has been patched to the latest patch. I have already completely replaced the stairs, removed everything around the stairs, but still no. This is not a problem with all the stairs, just with lots that I have saved in my library, and then try to use.