If your vehicle's handbrake is not working or holding the vehicle correctly, then make sure you check the following:

Cable adjustment 

Inspect the handbrake cables and lever mechanism. 

Ensure that lifting the handbrake lever will activate the handbrake mechanism at the rear wheels. 

If there is insufficient travel on the rear calipers to grip the disk, adjustment of the cable at the handbrake lever may be needed

Jammed or damaged cable 

Inspect the handbrake cables and handbrake lever movement. 

It is possible that the cables have been jammed, rusted or damaged. 

Replace if necessary.

Jammed or stuck caliper

Inspect the rear calipers and their movement. 

Replace if necessary.

Worn or damaged rear brake pads

If the rear brake pads are worn or damaged, this can cause the handbrake not to function effectively. 

Replace pads if necessary.