Sims 3 custom content problem?

how come some of my custom sims that i download show up in the sims bin when creating sims but only as an invisible sim. i try clicking the box and a invisible person shows up, theres no one there. also when i try to place a custom lot on to the pc it does not appear to show up. rather, it still stays an empty lot. when it does show up and i try to save the game it crashes. someone help plz.