Sometimes when an object is falling, all of the PE is not completely converted to KE. What happens to it?
The energy is transformed to other forms such as heat and sound.
If you were to drop a ball from a fixed height and all energy was converted from PE to KE then back to PE it would come back to the height it was dropped from.
However, this does not take into consideration factors such as wind resistance. The wind resistance is a form of friction which produces heat energy and sound. This is why you can hear a ball falling and why brakes get hot on cars.
The initial PE = KE + Energy lost to other forms.