One thing could be a routing error. It's where your sim can't reach whatever. The main cause for this is when the move objects cheat has been used. Go into build mode and take the item your sim is not going to and see if you get red instead of green. Red means something is overlapping, and it won't go into that spot. Move the item to where it shows green and set it down. Then go into play mode and see if your sim can reach the item. There are quite a few lots in the gallery where the creator has used the move object cheats, and not check for routing errors.