What Shallina said. Also, make sure that things like carpets and rugs are tagged as 'walkable'. Converting things laying on the floor, like rugs, vases and the like, to Environmental Objects means that they can now be walked right through and don't count for the walkmesh when baking.
Also, try to ensure your walkspaces are not to tight or cluttered. Pathing in Interiors can be pretty hard and the walkmesh is pretty finicky about having X amount of room with the polys it uses to determine the walkmesh. In some cases, simply moving an item a bit one way or another into a different poly or onto only a single poly (instead of covering more than one across the lines) can also help out when it comes to baking the mesh for walkable areas.
If that doesn't work, though, re-read what Shallina wrote, because there won't be anything else you can do about it other than completely remove the items causing the issue. This is especially troublesome near doorways and entry ways, where if you put something down, it may show the rest of the tile walkable, but you've got something blocking the one or two ways to actually enter that tile area.
Hope that's useful to you.
dunniteowl