When creating a drop-down menu/hover menu, the subitems will usually go as wide as the parent li. To make them go as wide as the value of their containing li you have to add a white-space: nowrap.

If you still run into issues you may have to make sure you've assigned display:block to the li that is wrapping also.

ul li li{
  display: block;
  white-space: nowrap;
}