The parking brake relies on a steel cable to apply the rear brakes mechanically. When that cable stretches or seizes, the parking brake misbehaves. Here are the symptoms of a bad parking brake cable.
A loose lever that won't hold
A stretched cable lets the lever or pedal travel too far without applying enough force, so the parking brake will not hold the car on a hill; see how the parking brake works. The lever may feel loose or come up too high.
A brake that won't release
The opposite problem is a corroded, seized cable that stays applied after you release the lever - the rear brakes drag and the wheel runs hot; see stuck parking brake. This is common on vehicles that sit or see winter salt.
Uneven or one-sided hold
If one cable is worse than the other, the parking brake applies unevenly - one rear wheel holds and the other does not. A dragging one-sided cable also causes a pull and premature wear on that corner.
What to do
Replace a stretched or seized cable and adjust the parking brake to spec; free or lubricate the mechanism if it is just sticky. Keep the rear brakes healthy with quality pads and hardware.