Zero y of figures & props.

I do have a problem. If I place a figure in the scene, it's usually placed on the floor. If not, I can correct via Y-translate-dial or shortcut. But if I add stuff like a prop to the scene sometimes the prop get's placed correctly on the floor but the figure floats above. Now if I try to manually correct Y of the figure, everything starts to float. What am I doing wrong. This is for a scene without specific environment. Thanks!